JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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.
Unfortunately, it is not possible to add a script in our form builder.
What you can do is to create your own HTML page where you can place your code there, then upload the HTML page on a hosting site, if you have a website page.
Then you can redirect to that page which will be use as the Thank You Page. Here's a guide on how to redirect your user to a different confirmation page: https://www.jotform.com/help/38-Redirecting-Users-to-a-Different-Page
Just to let you know, we also have a Google Analytics widget that allows you to track conversion and visits from your form. Let us know if you need more help.