When reviewing completion of a REST API Webhook, two URLs can be used to perform...

The two combinations of URL and associated parameters that should be used to validate the REST API are Endpoint URL with Access Token and Your Email Address and Identity URL with Client ID and Client Secret. These combinations will allow the user to perform an initial validation and make sure leads will pass correctly through the REST API Webhook. The Endpoint URL with Access Token and Your Email Address will allow the user to test the functionality of the endpoint service and verify that the leads can be submitted successfully. The Identity URL with Client ID and Client Secret will allow the user to obtain an access token for authentication and authorization purposes.

References: https://developers.marketo.com/rest-api/ https://docs.marketo.com/display/public/DOCS/Webhooks