Coupon for a billing form

Hi guys

I made a billing form for order a drawing portrait for taking the order details and uploading the image that the customer want from me to draw it

The problem is I want to make a coupon for this form but when I went to wix reference I didn’t found the best code for creating a coupon would you tell me the best code the marking a coupon for billing form

Can you describe this ‘billing form’ please. Are you using Wix Pay API or a payment gateway’s API coupled with user input elements for this billing form?

No I use the payment feature in wix form