Just follow the instruction on How to get Facebook App ID and Secret Key and pay attention in Step 6: 

https://wpforo.com/community/faq/how-to-get-facebook-app-id-and-secret-key/

Hi Support Team,

I have installed your Forum plugin on my Wordpress site and have set up my Facebook API Configuration.  I have followed both your and Facebook documentation (several times); however, when trying to log in using Facebook the following message appears: "The domain of this URL isn't included in the app's domains. To be able to load this URL, add all domains and sub-domains of your app to the App Domains field in your app settings."

I have raised this question with Wordpress support who also would like to advise that their log is showing the following warning message: 

PHP Warning: session_start(): Cannot start session when headers already sent in /srv/htdocs/wp-content/plugins/wpforo/wpf-includes/class-notices.php on line 16

Thanking you for your assistance,

Raine

You need to login to Facebook tools and add a domain with and without prefix. Example: --> www.yourdomain.com <-- ; ---> yourdomain.com <<- Simple, right? :)

Thanks Robert, I have removed both.  Sorry I didn't reply earlier, different timezones. 

Not related to facebook login:

Please remove this CSS code from your website. This code affects all form elements of your website and forum. It makes tons of design problems:

input {
     height: 60px;
}

Then remove this custom CSS code from Forums > Settings > Styles > Custom CSS Code textarea. I think you've added this because that was affected by CSS code I mentioned above:

#wpforo #wpforo-wrap input {
      margin-top: 20px;
}

Sorry, just for your reference.  I just removed the Facebook login using the forum settings and checked the page again and the Uncaught error with the SDK (screenshot above) has now gone. 

Thanks for trying.  I do appreciate your time even though there has been no solution.

I'm really sorry but I have no other clue for this issue.

I have deleted the APP over 10 times (not joking) and started again.  I have the same problem every time.  I have even had myself validated with Facebook thinking that might be the problem.  I have had Wordpress look over my site and they can find no conflict.

As I said before I have tried another social login and it worked.

They also had a return URL with a "=facebook" just as the example above by WP Native Articles.  I'm guessing this is what I need to find some URL that has something referring to Facebook in it.

I have spent the last week reading all the articles about it.

And again NO, forget about that URL. You should insert your forum URL. So I think you have some site specific problem, because other social login plugins don't work too. I recommend delete the APP you've created on Facebook and start creating it again. There are tons of articles and support topics related to this error:

https://www.google.com/search?client=firefox-b-d&q=Cant+Load+URL%3A+The+domain+of+this+URL+isnt+included+in+the+apps+domains

Yes, I read the article sofy linked to.  The bottom half you have screenshot is not relevant because, as per the article,

"UPDATE – March 2018
Facebook has been aggressively tightening security. Any new Facebook Login Apps create AFTER the beginning of March 2018 now have Use Strict Mode for Redirect URIs and Enforce HTTPS enabled by default and can no longer be disabled."

They also say (and where I'm stuck because it no longer allows you to just put your domain name in):

"it means that you now have to put the exact return URL into the Valid OAuth Redirect URIs input. Previously, with strict mode disabled, you could just put your domain name in and that would be enough. Now it has to be the exact URL that your return to once you’re authorised Facebook. This will be different depending on the App you’re using to connect to Facebook. If you’re using WP Native Articles it’s:

http://your-site.com/wp-admin/admin.php?page=wpna_facebook&tab=api&wpna-action=facebook_login