Before starting to set up the integration make sure you have a page on Facebook.
We will begin by setting up a Facebook application. Go to Facebook developers site:

After setting up the app we will add a Messenger product set up:

The next ste is to generate a token for the page:

Save the token in your credentials.local.yml in fb_token key.
Now, let set up the address which Facebook will interact with Nuntius. Click on
Setup Webhooks:

After that a modal will open for you:

Couple of things:
- The address is the address of the facebook webhook handler -
https://b9204872.ngrok.io/nuntius-bot/facebook - The token is not mandatory, it can be anything.
- Under
Subscription Fieldscheck all the boxes.
You'll get this if anything is OK:

Now, we need to set up the page as a subscriber to the events we selected:

Please note that for now, when this doc been written, you can't change the address of the webhook.