Add to Secondary Usergroup if answer is correct

[Sarcoth]

I'm using the Form Hack written by Abe1.

I've set up a form to ask the user one question. If they answer correctly, they'll be added to the Secondary Usergroup #12 and then be redirected to another page. If they answer incorrectly, they will be redirected back to the same page and can try to answer again.

So far, I have the form redirecting the user to the correct page depending on their answer, but I have failed on figuring out the code to get them added to a secondary usergroup.

If anyone has any idea on how to do that, please let me know.

Thank you.