Fasih, thanks for this awesome guide!
I got all the way to Step 15 and try to verify my webhook on the new Facebook page but am getting this following error:
“The URL couldn’t be validated. Callback verification failed with the following errors: HTTP Status Code = 502; HTTP Message = Bad Gateway”
I successfully subscribed the app to the page and am pretty sure I pushed the changes to Heroku in Step 14. In Activity on the Heroku dashboard I can see the App initial release and can subsequently open the app (i.e. get the URL).
Any advice on how to proceed/trouble shooting would be much appreciated!