Question about Guest Posting and Username Field

I allow guest posting on my forum and was wondering about something?

By default they must enter a name for themselves in the Username Field, is there not a way to have a default name in there such as "Visitor" already listed so they can write and submit the post without having to enter a guest user-name?

But of course they could remove "Visitor" and enter their own name if wishing too. Done really just to simplify things more for guest posters, they don't have to enter a user-name if not wanting too, post would be listed as written by: "Visitor".