@icloud.com as default

Hello,

Does anyone know how or if my @icloud.com address can be used as the default address for everything? I noticed that I can't use it as the default for Facetime, as I'd like to, or any other app. If I try to add the @icloud.com as my alternate email (and eventually as default) to my AppleID account, it says that it is already in use by another user. 😕 but that can't be possible because I own it, but can't use it?

Can anyone help?

Thanks!