How can I add a coupon code to user defined amounts collected through pay pal?

Hello,

I'm using pay pal pro widget to collect payments, and having amoutns passed onto a calculation field.
What is the best way to add coupon codes to this process?

Coupons within payment field are only available when sell products or sell subscriptions options are selected. 

Considering that you are performing calculation and passing it to payment field, you can also add coupon code field to your form using basic text box field. Then you can use conditional logic to set coupon codes and calculate discount. 

On my cloned form I have added coupon code field and grand total field. I have used Update/Calculate conditional logic to perform calculation. 

Conditional Logic Inserts Text/Calculation Into A Field

I have added two coupon codes, one with 20% discount and other with 50$ discount. I have set my conditions like this. 

Inside of payment field I also changed Get Price From option. Instead of total, like you have set, I am getting value from my new field grand total. 

Here is my demo form: https://form.jotformpro.com/71511000657949 

Coupon codes are test1 (20% discount) and test2 (50$ discount). 

Feel free to test it and clone it. 

Hope this will help. Let us know if you need further assistance.