The app is deployed using the Procfile, so you must see that your app runs via the command in the Procfile, this currently means running your app via gunicorn main:app.
To test a webhook through a simple curl request you can use curl -X POST -H "Content-Type: application/json" -d "{ \"data\" : \"test\"}" followed by a space and your URL, you can change the data used for testing, you must escape any quotes, or you can include data from a file using the @ symbol like -d @file.json.
You must make sure your webhooks works if you miss out GET parameters, see CONTRIBUTING.md for how to do this.