How do you automatically reinforce Commanders?

[sly1foxy]

Saw this from the developers but I can't figure out how to do it!?
"If you're fighting a war 30 hexes away, like many of us do, you can build things back in your cities, you can tell it to reinforce a specific commander, and it'll get there on its own. You don't have to worry about how it's getting there, the interruptions along the way, and everything else."
I'm on my 3rd game and I've never figured out how to do this...can anyone help???
Thanks!