How to get around "already taken" error when adding an account alias using an existing "@outlook.com" address.

I've created an email address with "@outlook.com" domain. I now want to add it as an account alias, so I can then make it my "primary" alias. I can't add the address an alias.

When I enter it as a pre-existing address, the entry is rejected stating that the address is already taken. It's taken by me. Why doesn't it go through a verification process to confirm my ownership?