Domain verification for user-generated subdomains in Facebook

We run a platform where users get their own subdomain (e.g. customername.example.com). This is similar to github.io, myshopify.com, or any of thousands of other services providing their users with a subdomain that hosts their own content.

We have verified the root domain example.com inside Facebook’s ‘Brand Safety’ -> ‘Domains’ area and this is displaying as verified, however, when I go in to the ‘Aggregated event measurement’ section it tells me the domain is owned by another account, which it is not.

When a separate Facebook account goes in to verify their own subdomain, it verifies successfully, but when he goes into the ‘Aggregated event measurement’ area, it shows the root domain and tells him that all the options are hidden because it’s owned by another user. He never added the root domain and only has access to/control over the subdomain.

How do you set up a domain – specifically inside of Facebook – so that customers can verify their own individual subdomains?