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If you are using the Payment tools, you can easily setup coupon codes there.
You can follow the tutorial link here, https://www.jotform.com/help/233-How-To-Add-Coupon-Code-in-the-Payment-Form. And you can also use the screenshots below as a guide.
Adding coupon code in Payment Tools (Purchase Order)
1. Choose a Payment tool widget.
2. Setup the appropriate fields and continue until you reach the coupon code setup.
3. Add coupon codes and customize them.
4. In your form it will automatically display a coupon field textbox. You can also adjust this by clicking the wizard setup.
Let us know if this helps.