How to link from Qualtrics to third party experiment and back again?

Hi there, I am currently trying to set up an experiment that will take the participant out of Qualtrics to an external task that we have created. On completing the task on the external site, the participant needs to return back to Qualtrics to complete a couple more questions.
So far, I've tried just using a hyperlink out of Qualtrics but I had trouble brining the participant ID through. I think this is something our web developer needs to configure on the destination site but I'm still uncertain about how to rejoin the Qualtrics survey at the point that they exited.
Someone has suggested that I look at API and intercepts but I have no idea about this, so would rather not if possible.
Any help would me most appreciated!
Thanks!
(I should mention that the participants will actually start and end in Sona, but we know the Sona-Qualtrics and Qualtrics-Sona link works, so it's just a case of getting the Qualtrics-External and External-Qualtrics bit sorted)