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I checked your form and it seems like you have already setup coupon codes on your payment integration.
You can follow the quick step-by-step guide below to add a coupon code:
1. First, open the payment wizard.
2. Click the "COUPONS" tab. Here you can add and manage your coupon codes. Click the "Create Coupon" button.
3. You can then setup the coupon code here. You can set the code itself, limits, discount percentage, etc...
I hope that helps.