I need to create an online form that the customer puts in the price. It will then be paid by Square.

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.