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.
It is possible to allow the user enter an amount in a text box field inside the payment wizard. You can do this under the Payment Type Options inside the payment wizard. You should set the payment type to "User Defined Amount".
Here's how it'll look like:
Please check this related blog post: https://www.jotform.com/blog/246-Create-JotForm-Payment-Forms-Using-Square
Hope that helps. Thank you.