How to show subject line for new messages in OS X Messages.app? Mavericks, if that matters. On the iPhone, there is a separate subject line before the main message, and on the Mac it shows (as on the phone) in bold at the top of the balloon. But when I want to make a new message on the Mac, I only have the body available.
As far as I can tell, you can't. I have looked everywhere for this feature.

I'd love for this answer to become wrong though.