The section you want will look something like:

api_token and the transfer_number, follow these instructions to get up and running:
Control flow

callback field in the API. If it is left empty, the call is ended once payment is complete.
Step 4: Webhook get-context (optional)
Hitting the get-context endpoint will share details about how the payment processor went. To see an example of what this looks like for Twilio, check out: Get Context