Pick an endpoint, then inspect request and response payloads.
https://api.acmecorp.com/webhooks/billing
https://hooks.slack.com/services/T01/B02/xyz
https://zapier.com/hooks/catch/123/abc
{ "id": "evt_1a2b", "type": "invoice.paid", "data": { "object": { "id": "in_8842", "amount_paid": 4900, "currency": "usd" } } }