Verify a Domain in order to Customize Link Posts for a Facebook Page

1. Go to Facebook Business Manager: https://business.facebook.com.

2. On the Business Manager Homepage, find and click on the Business Settings button.

3. In Business Settings, click Brand Safety > Domains > Add, where you should see the domain of your site that needs to be added.

4. After the domain has been added, please follow the following instructions from Meta on how to verify whether your domain has been granted permission to customize link posts: https://developers.facebook.com/docs/sharing/domain-verification 

5. If a domain has been verified successfully, we should see something similar to the screenshot below. 

6. Click on Add Assets (see above screenshot) to associate the desired Facebook Page to the now verified domain

If you see your page under Connected Assets, then you have successfully verified your domain and are good to go.