You’re wording is inaccurate, as a result your asking something different in your initial post.
Skip adding anything at that step of the new account registration process. I suspect the problem is .uk.com is a sub level tld.
.uk.com can be correctly added to your account once you reach the control panel. I added a test entry with garbage letters to my account to verify that. https://imgur.com/a/EcHtk