The command to add a radio message is (you've guessed it) setRadioMsg. Or something.
To remove a radio channel use setRadioMsg "null".
What remains are the trigger conditions. Depends on exactly what you want but the commands vehicle or in are likely to help.
For example, trigger:-
Condition: loon1 in apc1
On activation: 1 setRadioMsg "Bale out!"
Syntax not guaranteed. At all.