I had that issue with another custom domain I tried to add. Seems at some point I created a new Apple ID using that email address so it wouldn't let me add it. I've requested that the ID is deleted so will try again once it's complete.
Puntoboy
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
Last seen
User for
From
United Kingdom