It's kinda still tied up in development - I think we were hoping to get a 'link' option (alongside bold, center etc) that did all the hard work for you. But current syntax ...
Here's a link to Leora's msg #6 above as an example:
link to a message in this forum
Syntax is:
http://rpol.net/display.cgi?gi=7&ti=4866&msgpage=&show=all#6
gi is game index
ti is thread index
you can find both gi and ti by opening the specific thread and looking in the address bar in your browser
the number right at the end is the message in that thread to which you want to link
show=all argument is required otherwise link won't work once thread goes to multiple pages
doesn't work if the linked post has been deleted (obviously)