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.
To pass some value to some field over the URL, you would need to use URL pre-populating.
You can see more about it here: Prepopulating fields to your JotForm via URL parameters
Now, it is always good to know the name of the field that you are passing the value to, which you can do by checking this guide: How to find Field IDs & Names.
The there are 2 things to point out.
1st is that you will need to pass the value to some field which is then passing the value to the payment tool.
2nd is that many people will notice that if they change the value in the URL they are shown a different value, so you should be prepared to see these things happening.
Depending on your setup which the form is part of, this might be desired and welcoming.
Do let us know if you have any further questions in regards to this.