How to add a new site (a blog) from sign up

Hi.

I've seen some screenshots of "Create an account" where there is a field named

"Blog details" - "Yes, I'd like to create a new site" with a check box

If the new user mark this check box, the system will create a new blog with the address:

http://newblog.thesite.com (for example)

The question is how to create this check box.

thanks, Emilio