Do you simply need to record the coupon code in the form response? Or do you need to do any extra processing in the form when the code is entered? Please provide more details. For a simple record, you may use a text field for that coupon code.
I need it to also change the price of the order... So when they entered the
code I want to charge -1.00 but I was unable to assign a negative amount
inn the collect payment fields