It's all triggers: you need to read a good general tutorial on triggers and also the one specifically on switch triggers.
In the Tutorial Mission (tutes - Getting started) there is a radio call for chopper CAS: in that case it's trashing a tank rather than picking somebody up, but the principle is the same.
Getting choppers to do exactly what you want is tricky - they are just the awkward bastards of OFP. Fortunately there are specific tutes on this too.
As an alternative, use snYpir's SP/MP support pack. Can't remember if it does chopper extractions, it probably does. In the Ed Depot under mods, on the left hand menu column.