Hi fernandorigotti
If you go to the Instant Articles tab in your Page settings, under the Tools heading you’ll see a section called ‘Connect Your Site’. There will be a meta tag there which will look something like this:
<meta property="fb:pages" content="1074570482568345" />
It’s the long number you want. That will then be auto added to your site and enable you to authorise your domain with Facebook.
The full Facebook guide is here: https://developers.facebook.com/docs/instant-articles/guides/publishertools
Hope that helps
