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.
You can add a "Hidden Box" in your form which can be found under "Widgets" tab:
Once you have hidden box in your form, you can use URL parameter feature to populate the hidden box with a value. This guide should help you understand url parameters: https://www.jotform.com/help/71-Prepopulating-fields-to-your-JotForm-via-URL-parameters
I have created a demo form for you. Take a look : http://form.jotformpro.com/form/40162880274958?userId=12345
This form has a hidden field named userId which I am populating through url parameters. You need to refer the guide to understand the naming of the form fields to use in url parameter. URL parameter is something very similar which you use in ASP.NET and call it query string.
Feel free to clone this form to take a closer look. This guide should help you in form cloning: https://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL
Hope this helps.
Do get back to us if you have any questions.