Actually, to make sure that you actually pick up the link for the post (as opposed to the link for the thread, or the link for the last post posted) you should go to the actual post, click edit, and then click save edit. When it saves, it will pop up the individual post address in the address bar at the top of the page. You can use that for your link.
As for not opening up in a new window, I'm not sure you as the poster can control that, as it depends on the reader's settings, both on site and in the browser.
Mine pop up in new windows or new tabs, depending on how I have IE set, and nobody has ever complained about that.