IMPORTANT #1: The Coupon Code is the ID not the Name
The coupon code is the code you enter into the ID field and not the Name field. See image below. An easy solution is to simply give an identical Name and ID.
In the above example the coupon code is "COUPON100" which you enter into the ID field which is marked as optional and not its "Name" field.
We suggest simply giving an identical Name and ID to keep things simple,
IMPORTANT NOTE #2: Turn off Google Captcha
You must turn OFF Google Captcha for SuperForms to work. If you turn ON Google Captcha, the Weebly Form will not submit. See image below with Google Captcha turned OFF. This is the correct setting for your Weebly Form if you are using SuperForms.