Where do I get a webhook url for a facebook messenger bot? I want to check out and explore this interesting drupal module made by the white house: https://github.com/WhiteHouse/fb_messenger_bot

I've successfully installed drupal on my Mac thru MAMP. I've also enabled the said module. But I got stuck on this step of the installation instructions as stated in the README:

"Complete steps 1-4 outlined in Facebook's quickstart documentation including the copying of the page access token in step 3 (note: this module sets up the webhook at /webhook/contact)"

To be more specific, Facebook is asking for a callback url, but I can't find it. Whenever I type https://localhost:8888/webhook/contact as the callback url, Facebook gives me this error message:

The URL couldn't be validated. Callback verification failed with the following errors: curl_errno = 56; curl_error = Received HTTP code 502 from proxy after CONNECT; HTTP Status Code = 502; HTTP Message = Private address

I am completely new to drupal and MAMP. Any help that would point me towards the right direction would be greatly appreciated. Thank you very much!
If you are working in a local development environment, you will need a publicly accessible IP address or domain name to route the FB request to your local server messenger bot webhooks.