I think he means to actually type something, say "Hey, we need some air support over here!" and have his player say that, and then have some planes or gunships come on over and help you out. I don't see how that's possible. Just use radio commands or other triggers. You can always have your player (or any other unit) say something over the radio using this command:
(unit name) sidechat "Here's what you'll see on the screen, in blue letters"
There are no () around unit name when you type it, though, just the name. I just wanted to seperate that from the rest.