Go to the link you want to reference. I prefer to open another instance of my browser so my posting page stays in place. Highlight the URL/Address and hit <Ctrl>C.
Go back to your post and hit the markup option "URL" (located below the posting dialog box.) At the first prompt box hit <Ctrl>V, which enters the address. In the next prompt box, enter the title of the link you want to reference. If you're previewing posts, you can check your link to make sure it worked.