Create a script file named "init.sqs" in your mission folder (Desktop\Documents\ArmA other profiels\yourProfiel\missions\yourMission.sara). It's a simple text file so you can edit it with notepad. Just add the following code
(it's for radio alpha, if you're using another channel use the proper number, see the link added by Wolfrug)
1 setRadioMsg "NULL"
Then when the proper condition is satisfied create a trigger with the required condition and in the activation field write 1 setRadioMsg "Whatever message you please"