The address of each thread is displayed in the field where you type in web page addresses. It actually says "address". When you are viewing a thread, note the address in the address box. If you need to reference it in another post, simply highlight the address, then right click over top of it. Select "copy". Go to the new thread (where you want to reference the previous thread) and right click again, and select "paste". This will carry over the linked address for the previous post.
My description sounds a lot harder than it really is. If you don't understand my wordy response, let me know!