How to generate an external link using only a URI?

I have a field in the user account form where the user can enter their Twitter username, i.e. without the http://twitter.com/ part on the front. I want the user account page to render the username as a link i.e. http://twitter.com/username and I imagined I could do this with a "link" field by putting http://twitter.com/ into a relevant field and then adding the [user:field_twitterusername] token after it. But when I try to do this, on display the link becomes http://www.mysite.com/twitterusername instead. (It seems to be thinking that "twitterusername" is an internal URI.) Maybe I'm just misunderstanding the "link" field, in which case is there any other way to get this to work?