linked thread - I assume you mean a link similar the following which slowhands has done on his posting.

http://www.forum.forest-of-dean.net/index.php?mode=thread&id=9525#p9527

If that is the case you need to first of all go to the thread you want to link to, which is usually done by using the search facility of the forum. When you have found the relevant posting you wish to link to you then copy that link (URL) which you see in the Address bar at the top of the page and pasting the address (URL) in your post.

Best Wishes
Chris