You need to use the 'send transmission' trigger. By default, it looks like this:
Send transmission to [All Players] from [Unit] named [Name]: Play [No Sound] and display [Message]. Modify Duration: [Add] [0.00] seconds and [Wait]
[All Players] - usually best to leave this as-is, but if you only want some players to see it, then feel free to change it to just those players.
[Unit] - You need to select the unit that will do the talking. That unit's selection circle will flash white and that unit's portrait will show up next to the speaking text.
[Name] - Type whatever you want. This is the name that will come up next to the portrait, above the speaking text.
[No Sound] - If you have a sound file you want to use for the speech, select it here. Otherwise leave it as 'no sound'.
[Message] - This is where you type in what you want to appear in the speaking text.
[Add] [0.00] - Here's where you modify the amount of time the transmission stays up in the cinematic - be sure to make it long enough that people can read it. I think the default is 5 seconds, and you can 'add' or 'subtract' from that, or just 'set' it to a certain duration.
[Wait] - If you select this, then no other triggers will go off until the 'wait' duration is over. If you select 'don't wait', then other triggers can still go off (you'll need this to be on 'don't wait' if you want to, for example, switch to a different camera during the middle of someone's transmission or anything like that).