Your problem is likely because there was no response data. This can happen based on the http status code. You can echo $urlget(%id).reply to see the headers.
But I think I've identified a problem using $urlget for this purpose. When you finally get the correct POST sent, the response is a 302 redirect which mIRC follows and eventually responds with a 200 OK. But the original response headers with the 302 are no longer available so you cannot retrieve the auth cookies. It took me a while to realize this was the problem rather than something I was doing wrong.
mIRC needs to add the ability not to follow redirects; I could imagine other ways to work around this but that's probably the easiest. Until this is added you'll need to use raw sockets, use curl to obtain these cookies, or use another script like JSON-For-Mirc
Edit: Looks like jsonformirc also follows redirects after a POST?