How do I determine the payment field coupon code

I have a registration form with a PayPal payment at the end.  The payment can be either a Member Payment or a non-member payment.  I would like this to have a payment required to submit registration.   I could set up a member discount as a coupon.  But how do I determine the coupon code?  I could be their membership number--but how is that defined?

To make the payment field required , click on the properties button and set Required Field option to YES

As for creating coupon code check our user guide: How to Add Coupon Code to Payment Forms