In your OTT site admin, navigate to Manage > Site.
Select the Tracking tab. In the Facebook section, you will see a Domain Verification input. In order to get the proper value for this field, you will need to go into your Facebook Business Manager and follow the instructions outlined here. Once you have a metadata tag it should look something like this:
<meta name=”facebook-domain-verification” content=”123abc”>
You will want to copy only the content value (e.g. "123abc" above) and paste it in the domain verification code input. Be sure to save your settings.
Adding this value will automatically add a metadata tag to the <head> section in the HTML of your site's homepage. You can then click Verify at the bottom of the Meta Tag Verification tab in your Facebook Business Manager and Facebook will be able to successfully verify your domain.