When you post a reply or start a new thread simply enter the song/title/whatever into that small now playing box. When you post your message the now playing information will be displayed in that thread only.
Actually I've just noticed a small bug I think. If you leave the box blank with your first post to any chosen thread, you will not be able to enter a now playing song for the rest of the thread. This shouldn't be happening. We'll work on it shortly.