URI help

[eluser]allancaps[/eluser]
Hi, this may have been asked, but i can't seem to find the thread, so I'm asking this again... How can I make the URI to become "myusername.mydomain.com" when the user creates an account using "myusername" as the account name?

I'm only stuck at being able to do mydomain.com/member/userid as my URI...

TIA!