Rudi provides a thorough discussion in the CBOK (see below link):http://farmallcub.com/phpBB2/viewtopic.php?t=17556
But to answer your question directly, you simply go to the post you want to reference and copy the url address (this can easily be accomplished by placing the cursor in the url address at top of that page, right clicking it and then select copy from the drop down menu. Immediately after that you place the cursor in your reply post and right click again. Then you select paste from the drop down menu. To insure that the link work properly, you can select preview of your post before submitting it. Hope that helps.