The script on this page is able to extract GET parameters. One could use this HTML page to redirect to some other page, e.g.
Note that there is no & in the target URL. This means that the new participant_label will be correctly handled by oTree (which should presumably live under the target URL). The function targetURL() can conveniently construct the intended link. Click here to see another example and observe how the target URL changes.
Instead of printing it on the page as done here, you could redirect to it using location.href = targetURL();.