First, go to developers.facebook.com. Click on the menu in the top right corner and select Add a New App.
Name your app, enter your email address and press create.
Select the product Facebook Login by clicking 'Set Up' on the Facebook Login card.
Choose the platform 'Web'.
Enter your authentication domain (something.cloudflareaccess.com) and click save then continue.
Copy the App ID and App Secret into the Cloudflare dashboard into the fields called OAuth ID and OAuth Secret respectively.
Then click on the Facebook Login option in the left side menu. Move the slider to Yes for the option 'Use Strict Mode for Redirect URIs'.
Then enter into the Valid OAuth redirect URIs field: your authentication domain /cdn-cgi/access/callback. Click Save Changes.