Adding TXT record with name "@"

Hello.

I think you can only create a subdomain or naked domain of "mydomain.com" within the hosted zone of "mydomain.com".

Perhaps what you want to do is create a TXT record with a naked domain?
In that case, you can register by leaving the "Record name" part blank.

Thank you very much for your support! It is actually working now, leaving the name field blank as you suggested. Thank you!