In the recent past, Facebook is blocking unverified domains from sharing content to there pages or in groups.
And in this tutorial, am going to show you step by step on how to verify your website domains with Facebook to get that issue sorted. All you need is a Facebook account and also join the business.facebook.com platform.
To Read: How to Promote A Facebook Page For Free (Boost Facebook Page)
That way, you will be able to add and verify your domains. Follow this tutorial for a walk through to have your domains/websites verified on Facebook.
PS: Apologies as i have had to blur out some areas for security reasons. Hopefully you can be able to verify your domains with no hinderance. Let me know in the comment box your feedback or suggestions.
Below is the video tutorial.