When Saigon began to fall on April 29, 1975, they signaled the start of the US evacuation by playing White Christmas (as sung by Bing Crosby) on AFRN:

https://en.wikipedia.org/wiki/Fall_of_Saigon#Operation_Frequ...

I guess these days, it'd be done by a smartphone app (some sort of secure Telegram group or emergency broadcast type message).