Hey man... ! it was hard to understand you... But i think you wanna a trigger that's activated when a soldier gets into it, and, consequently, it sets a radio message to the alpha channel...
Try this:
When trigger1 is activated, at the init field you'll have 'channelnumber setradiomsg "message"'
ex.: 1 setradiomsg "Airstrike"
(Hey, check the command and syntax before you use it, as i don't have a CommandReference right now...!)
As the empty channel Alpha has now a message on it, you'll be able to use it for activating a trigger (which follows the instructions at the init field). Remeber that you must firstly set a trigger activated by the alpha channel, whose message is set to "null".
Well, that's it. I hope i didnt get wrong your question...