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.
Have you tried navigating to "Installations"?
You should be able to find the option "Payment Response URL" and add this URL http://www.jotform.com/ipns/worldpay.php. You'll also need to enable the check box "Payment Response enabled".
If you are having a hard time finding it, you can check with Worldpay support team to directly assist you with their platform. They might have made some changes on the interface.
Let us know if you were able to make it work.