After establishing and maintaining Facebook Business Manager, verifying the domain name is required. You're able to do so by selecting Brand Security. Add your domain to the web page, then click Add.
You can refresh the page to confirm the domain name. It will then be confirmed for the domains. It is possible to validate a domain with Metatag Verification, HTML File Upload, and DNS verification. I recommend DNS validation because this is very straightforward. Please see the next link in the menu. Just log in at a domain registrar and go into DNS.