Registration Form, "Your Site /" Field

I'm trying to get the site registration to work w/ a form plugin. Can't find one that let's you create a professional looking form that also has the "Your Site /" field already built into it.

Do you know the form data / meta data needed to add this field to a form?

AR Forms has an impressive user sign up add on but it doesn't have the site registration field.

Are there any plugins that let you create professional looking forms that have that field as a builtin option?

Thank you.