This thread might help you…
Or perhaps this more recent thread…
Although, I’ve never been able to get anything like that to work. Also, there are security issues which result in browser differences, settings, plugins, firewall and AV programs which can block attempts to access the current or previous page URL.
It might be better to just ask the participants to manually enter an ID in your prototype, like on a landing page. When your tasks are finished, you could have a “jumpoff” page in your prototype which generates and shows them a code they can then enter or provide to the third party site for verification of completion and payment. Or, if you have a limited set of users and can get the third party to assign specific IDs (or share that set of IDs with you) you could create unique landing pages, one per user, so the URLs to your prototype might look something like, mysite.com/test5/1001 and mysite.com/test5/1002 etc.