What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I can see that you have already managed to add a discount field to your form, with conditions to calculate the discount:
However, this Discount Code field is currently hidden on your form, so your users will be unable to enter their discount code. To make it visible to your users, you would need to click on this field's gear-shaped icon, and select Show field.
Please let us know if you need further assistance, we will be happy to help.