There are two ways actually,
One is:
Unit SideChat "Your Text Here"
Another is:
Unit SideRadio "Your Text Here"
The difference is that SideRadio has to be a recorded sound which you make with GoldWave or something.
SideChat will make no sound at all.
If you want just the plain white text on the screen, put:
TitleText ["Your Text Here", "Plain Down"]
Beware the GrimMonkey