10% Off for Telling Us where you heard...

Hey all, first post. Zencart 1.3.7 user.

I am creating a 10% off for the buyer telling us where they heard about us. I have it setup in coupon admin for a 10% off coupon, I have the product in a category called, Other, product name 10% Off Discount!, required attribute of a text box, max. value 1 coupon.

So the buyer can add it to the cart, but what should the attributes be so it takes 10% off? how can i get it to show the 10% when another item is in the cart? so i put a $30 item in the cart, and the 10% coupon, how can the total be shown in the cart of $27?

Thank you, Any help would be greatly appreciated!

-d