Got it figured out. It was enough to copy/paste the form PunBB was generating. It didn't work out for me the first time because I probably missed some hidden fields, or my CMS changed the form slightly.
Thanks for your time 
1 2008-10-02 14:54
Re: Posting from an outside form (1 replies, posted in PunBB 1.2 discussion)
2 2008-10-02 12:56
Topic: Posting from an outside form (1 replies, posted in PunBB 1.2 discussion)
Hello all,
I am trying to integrate PunBB into my site, and I want to allow guest users to post their message from a form on the website homepage (which is outside the PunBB installation, but on the same domain name).
How should I go about generating this form, or rather, the script that would add a post to the forum? Is there a direct way or do I need to write a script?
I was looking at both post.php and common.php and couldn't find a function that would simplify this task.
Thank you all and I apologize if this was answered previously. I would be grateful for documentation hints as well!