How do I reply to a message?
Click Reply to respond to a particular post.
The Reply Message screen is similar to the Post Message screen, with these differences:
When you reply to a post, the subject line is filled in automatically. You can change it if you want.
You can paste the message to which you are replying into the body of your reply by clicking Quote Message.
You reply is added the existing thread; it won't create a new thread.
For more information see here: Posting Messages