Coupon Code with user defined amount

I am creating a form where the total price uses advanced calculations based on the options and responses the user chooses.  I would still like the ability to offer a coupon code that would say take a certain amount off the total.  In my research, I only find the ability to use coupon codes when the payment is set to sell something.  Can you help me with this?

Thanks

On user defined amounts, The coupon code will no longer apply because the field will only accept the full total of the payment to be made. You can instead create your own coupon code by adding your own short text field in the form. Then create another condition that will run a calculation for a discount.  

Just make sure that you have another field that will give a zero result or the discount value result, so that when ran under the calculation widget, it will not malfunction on its computation. 

If you need assistance, please let us know by giving us details on the form where it should be added and under which total it should apply.

Thanks.