I set up my @icloud.com email address on my iPhone. It suggests available alternatives. Once you migrate your own domain, you can just use that instead, and never have to use the @icloud.com address for anything externally visible.
I followed the instructions for setting it up on iPhone and iPad too but there’s no option to switch it on there on either device. If I search for it in settings, it shows up in the results but tapping that takes me to the place where it should be. https://twitter.com/drcongo/status/1484592787634937858
drcongo|4 years ago