XF 2.2 Prefill custom user field on registration form from query string param?

[lobanz]

Looking for the best way to prefill a user custom field value on the registration form from a query string param. Idea is that I have a long-ish code for them to enter and would like to put it in a QR code that I pass out on slips of paper. I'm doing this in an addon. I looked in the XF registration code and saw a bunch of encryption stuff so I figured I better ask.

Any idears?

Thanks.