Just paste the URL from your address bar in for the simplest link, it'll do it automatically.
To actualy link words (so they go green qnd become clickable), select them and select the Link icon from the toolbar - it looks like two chain links stuck together, and it's located after the text formatting stuff.
By the way, duplicating threads is not strictly allowed - if you don't have anything new to add to it, and just want to revive it, duplicating it is basically a greater form of SPAM. If you do have something to add to it, and it's on pages 1, 2, or 3 of its board, just reply to add your thoughts. If it's after page 3 and you have something to add, creating a new thread is fine.