Well you can create the trigger normally and if it's Alpha you can disable it until your ready to use it.
Disable
1 setRadioMsg "NULL"
Enable
1 setRadioMsg "working"
The problem is when you do enter the trigger and enable the radio it will be enabled for all Group Leaders at the same time. Radios cannot be locked to one player. The only other way I know of is to forget the radio and just use addaction in a trigger
Anyone
present
Repeating
cond
player in thislist
on act
act = player addAction ["Use Radio","dothings.sqf"]
on dea
player removeAction act
Addaction can also be placed in an objects init directly and has other options.