JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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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.