Mail-in form....or mail in form?

Question

We use a "Mail-In Registration Form." Or, should we be using a "Mail In Registration Form," without the dash?

Answer 1

The dash must be there.

"Mail-In Registration Form" means that form is sent over the mail.

"Mail In Registration Form" means two entirely different things:

1. An imperative command to mail that form in
2. A statement that there is mail contained in the form.