You should be able to get the custom fields in the Get Subscription API and Webhooks

In the subscription object you should be able to fetch the custom fields values that you've attached to your created subscription.

Follow Guide: Get Subscription Endpoint

Here's a sample response of the get subscriptions endpoint:

{
    "data": [
        {
            "id": 8768,
            "type": "subscriptions",
            "attributes": {
                "merchant_id": 202,
                "customer_id": 3106,
                "payment_type_id": null,
                "bank_id": null,
                "shipping_method_id": null,
                "payor": "Customer Name",
                "billing_address": "Address",
                "billing_province": "Province",
                "billing_country": null,
                "billing_city": "City",
                "billing_barangay": "Barangay",
                "billing_zip_code": "1234",
                "recipient": null,
                "shipping_address": null,
                "shipping_province": null,
                "shipping_country": null,
                "shipping_city": null,
                "shipping_barangay": null,
                "shipping_zip_code": null,
                "other_info": [
                    {
                        "id": 141,
                        "code": "otherinfo",
                        "label": "otherinfo",
                        "value": {
                            "order_no": 12345678,
                            "awb_number": "ABC123",
                            "delivery_number": "ABC1234"
                        },
                        "data_type": "json",
                        "is_visible": false,
                        "is_required": false,
                        "merchant_id": 202,
                        "dropdown_selection": []
                    }
                ],
                "payment_schedule": null,
                "max_payment_count": null,
                "last_billing_date": null,
                "shipping_fee": null,
                "custom_shipping_fee": null,
                "bank_fee": null,
                "vat_amount": null,
                "convenience_fee": null,
                "original_price": null,
                "total_price": null,
                "total_amount_paid": 0,
                "reference_id": null,
                "success_redirect_url": null,
                "failure_redirect_url": null,
                "paymaya_wallet_customer_name": null,
                "paymaya_wallet_mobile_number": null,
                "paymaya_link_id": null,
                "paymaya_payment_token_id": null,
                "paymaya_verification_url": null,
                "paymaya_card_token_id": null,
                "paymaya_card_type": null,
                "paymaya_masked_pan": null,
                "brankas_masked_pan": null,
                "is_api_booking": true,
                "is_checkout_booking": false,
                "completed_at": null,
                "cancelled_at": null,
                "created_at": "2022-07-26 18:40:46",
                "updated_at": "2022-07-26 18:40:46",
                "deleted_at": null,
                "total_price_updated_at": null,
                "is_console_booking": false,
                "is_deep_link_checkout": false,
                "is_shopify_booking": false,
                "is_membership": false,
                "is_auto_charge": false,
                "delivery_note": null,
                "voucher_id": null,
                "subscription_import_id": null,
                "discord_user_id": null,
                "schedule_email_id": null
            }
        }
    ],
    "links": {
        "first": "https://api-sandbox.helixpay.ph/v1/subscriptions?page=1",
        "last": "https://api-sandbox.helixpay.ph/v1/subscriptions?page=1",
        "prev": null,
        "next": null
    },
    "meta": {
        "current_page": 1,
        "from": 1,
        "last_page": 1,
        "links": [
            {
                "url": null,
                "label": "« Previous",
                "active": false
            },
            {
                "url": "https://api-sandbox.helixpay.ph/v1/subscriptions?page=1",
                "label": "1",
                "active": true
            },
            {
                "url": null,
                "label": "Next »",
                "active": false
            }
        ],
        "path": "https://api-sandbox.helixpay.ph/v1/subscriptions",
        "per_page": 10,
        "to": 1,
        "total": 1
    }
}

Also, Custom Fields values are available in the Webhooks

See Full Webhooks Guide:

Sample webhook post body:

{
   "event":"subscription.created",
   "data":{
      "id":8770,
      "merchant_id":202,
      "customer_id":3108,
      "payment_type_id":null,
      "bank_id":null,
      "shipping_method_id":null,
      "payor":"Customer Name",
      "billing_address":"Address",
      "billing_province":"Province",
      "billing_country":null,
      "billing_city":"City",
      "billing_barangay":"Barangay",
      "billing_zip_code":"1234",
      "recipient":null,
      "shipping_address":null,
      "shipping_province":null,
      "shipping_country":null,
      "shipping_city":null,
      "shipping_barangay":null,
      "shipping_zip_code":null,
      "other_info":[
         {
            "id":141,
            "code":"otherinfo",
            "label":"otherinfo",
            "value":{
               "order_no":12345678,
               "awb_number":"ABC123",
               "delivery_number":"ABC1234"
            },
            "data_type":"json",
            "is_visible":false,
            "is_required":false,
            "merchant_id":202,
            "dropdown_selection":[
               
            ]
         }
      ],
      "payment_schedule":null,
      "max_payment_count":null,
      "last_billing_date":null,
      "shipping_fee":null,
      "custom_shipping_fee":null,
      "bank_fee":null,
      "vat_amount":null,
      "convenience_fee":null,
      "original_price":null,
      "total_price":null,
      "total_amount_paid":0,
      "reference_id":null,
      "success_redirect_url":null,
      "failure_redirect_url":null,
      "paymaya_wallet_customer_name":null,
      "paymaya_wallet_mobile_number":null,
      "paymaya_link_id":null,
      "paymaya_payment_token_id":null,
      "paymaya_verification_url":null,
      "paymaya_card_token_id":null,
      "paymaya_card_type":null,
      "paymaya_masked_pan":null,
      "brankas_masked_pan":null,
      "is_api_booking":true,
      "is_checkout_booking":false,
      "completed_at":null,
      "cancelled_at":null,
      "created_at":"2022-07-26 19:27:50",
      "updated_at":"2022-07-26 19:27:50",
      "deleted_at":null,
      "total_price_updated_at":null,
      "is_console_booking":false,
      "is_deep_link_checkout":false,
      "is_shopify_booking":false,
      "is_membership":false,
      "is_auto_charge":false,
      "delivery_note":null,
      "voucher_id":null,
      "subscription_import_id":null,
      "discord_user_id":null,
      "schedule_email_id":null,
      "customer":{
         "id":3108,
         "merchant_id":202,
         "shopify_id":null,
         "name":"Customer Name",
         "email":"[email protected]",
         "country_id":175,
         "new_country_id":null,
         "mobile_number":"9123456789",
         "formatted_mobile_number":"+639123456789",
         "new_mobile_number":null,
         "address":"Address",
         "country_name":null,
         "province":"Province",
         "city":"City",
         "barangay":"Barangay",
         "zip_code":"1234",
         "viber_info":null,
         "other_info":[
            
         ],
         "created_at":"2022-07-26T11:27:50.000000Z",
         "updated_at":"2022-07-26T11:27:50.000000Z",
         "deleted_at":null,
         "is_unsubscribed":false,
         "discord_user_id":null,
         "discord_user_username":null
      },
      "products":[
         {
            "id":9944,
            "subscription_id":8770,
            "product_id":null,
            "product_variant_id":null,
            "option_values":null,
            "title":"Test Product",
            "description":null,
            "images":null,
            "shopify_custom_links":null,
            "payment_schedule":{
               "day":1,
               "frequency":"monthly"
            },
            "max_discounted_order_count":null,
            "price":null,
            "discounted_price":null,
            "total_discounted_price":null,
            "quantity":1,
            "total_price":null,
            "group_number":1,
            "are_multiple_orders_allowed":true,
            "created_at":"2022-07-26 19:27:50",
            "updated_at":"2022-07-26 19:27:50",
            "is_shippable":false,
            "is_membership":false,
            "shopify_product_info":null,
            "sku_meta_notes":null,
            "is_active_discord_member":false
         }
      ]
   }
}