Paypal webhook not contain custom_id - paypal

Question

my code can generate a link for subscription and it work fine but when it send data to my webhook all field contained in the data except the custom_id.
Please advise how can I resolve this?
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://api-m.sandbox.paypal.com/v1/billing/subscriptions');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, '{
"plan_id": "P-97A61885CU957844JMFV",
"start_time": "2021-10-19T00:00:00Z",
"shipping_amount": {
"currency_code": "USD",
"custom_id": "xxxxdxxx",
"value": "10.00"
},
"subscriber": {
"name": {
"given_name": "John",
"surname": "Doe"
},
"email_address": "customer#example.com",
"shipping_address": {
"name": {
"full_name": "John Doe"
},
"address": {
"address_line_1": "2211 N First Street",
"address_line_2": "Building 17",
"admin_area_2": "San Jose",
"admin_area_1": "CA",
"postal_code": "95131",
"country_code": "US"
}
}
},
"application_context": {
"brand_name": "SAMPLEBRAND",
"locale": "en-US",
"shipping_preference": "SET_PROVIDED_ADDRESS",
"user_action": "SUBSCRIBE_NOW",
"payment_method": {
"payer_selected": "PAYPAL",
"payee_preferred": "IMMEDIATE_PAYMENT_REQUIRED"
},
"return_url": "https://sample.com/webhook_paypal.php",
"cancel_url": "https://sample.com/webhook_paypal.php"
}
}');
$headers = array();
$headers[] = 'Content-Type: application/json';
$headers[] = 'Authorization: Bearer '.$token;
$headers[] = 'Paypal-Request-Id: SUBSCRIPTION-21092019-001';
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$result = curl_exec($ch);
if (curl_errno($ch)) {
echo 'Error:' . curl_error($ch);
}
curl_close($ch);
echo $result;
Here's webhook
{"id":"WH-82854424W5814533A-78W64324N5754181T","event_version":"1.0","create_time":"2021-10-20T03:35:20.009Z","resource_type":"subscription","resource_version":"2.0","event_type":"BILLING.SUBSCRIPTION.CREATED","summary":"Subscription created","resource":{"shipping_amount":{"currency_code":"USD","value":"10.0"},"start_time":"2021-10-21T00:00:00Z","quantity":"1","subscriber":{"email_address":"customer#example.com","name":{"given_name":"John","surname":"Doe"},"shipping_address":{"name":{"full_name":"John Doe"},"address":{"address_line_1":"2211 N First Street","address_line_2":"Building 17","admin_area_2":"TELEGRAM123","admin_area_1":"CA","postal_code":"95131","country_code":"US"}}},"create_time":"2021-10-20T03:35:19Z","links":[{"href":"https://www.sandbox.paypal.com/webapps/billing/subscriptions?ba_token=BA-91B88793MP435642P","rel":"approve","method":"GET"},{"href":"https://api.sandbox.paypal.com/v1/billing/subscriptions/I-SYPXUU0207HG","rel":"edit","method":"PATCH"},{"href":"https://api.sandbox.paypal.com/v1/billing/subscriptions/I-SYPXUU0207HG","rel":"self","method":"GET"}],"id":"I-SYPXUU0207HG","plan_overridden":false,"plan_id":"P-1EC1893707675350SMFWSTEI","status":"APPROVAL_PENDING"},"links":[{"href":"https://api.sandbox.paypal.com/v1/notifications/webhooks-events/WH-82854424W5814533A-78W64324N5754181T","rel":"self","method":"GET"},{"href":"https://api.sandbox.paypal.com/v1/notifications/webhooks-events/WH-82854424W5814533A-78W64324N5754181T/resend","rel":"resend","method":"POST"}]}

Answer 1

For a subscription to be approved, a payer account needs to subscribe.
A legacy integration method is to use the href with rel:approve that is returned from the API call.
The proper modern integration method is to use a JS SDK button, as documented in the Subscriptions Integration Guide.
Subscription creation can be either done directly in the JS (as that guide documents), or alternatively the JS createSubscription function can call a route on your server which returns the subscription ID from API creation. (The approve href is not used in this case; redirecting away from your site is bad).