Can't create custom domains with self-managed certificates


I can’t create custom domains using self-managed certificates (I have a B2B paid plan).

This is what the UI shows (the self-managed option is missing):

If I try through the Management API, I get the following:

    "error": "Forbidden",
    "message": "The account is not allowed to perform this operation, please contact our support team",
    "errorCode": "operation_not_supported"

The API Explorer Application has the permission create:custom_domains. Listing custom domains works (so not an authentication issue).

I can’t find any prior requirement in the guide other than having a paid plan.

How can I solve this?