How do you specify the requester's workerID in a mechanical turk HIT?

Question

I want to create an mturk HIT that has a URL like so:
www.example.com?source=worker_id
where worker_id is the worker's ID code. I'm initially going to create these from the mturk web UI, then once I get it working right, from PHP. But I can't figure out how to get at the worker's ID from the modified-HTML syntax of an mturk HIT.