We have configured a new Facebook routing account in Adobe Campaign Sandbox instance by following the online documentation.
When we click Request authorization from the application, getting the following error
"A Facebook error has occurred.
URL could not be loaded: The domain of this URL has not been added to the domains of this app. Add all your domain's and subdomains to the App domains field in your app's settings to load this URL."
We have also added the domains in Facebook app but still the issue is not resolved.
Which version / build of Adobe Campaign are you running on your sandbox? As Facebook's API regularly changes, usually the latest version of Campaign is required to have these API changes taken into account by Campaign.
Have you tried following this documentation? Publishing on Facebook (use) and Publishing on Facebook (configuration). You already mentioned it I know but I think it has been recently updated. So double-checking just in case.
Also, are you using HTTPS?
If that does not help, I'd recommend reaching out to our support team.