You don't. Under NO CIRCUMSTANCES WHATSOEVER.
Never use player waypoints to make things happen. Players in Veteran mode don't see the waypoints, so they don't even know its there. Plus you don't really know what Cadet mode players are up to. In other words, if you try and make something happen from a player waypoint, it probably won't happen at all.
Use a trigger instead.
Activation: player present (group the player to the trigger, you get new options in the On Activation box)
Type: End#1
Condition: this and obj1
obj1 is a variable which is set to true with the command
obj1=true
when the Objective is completed. Read snYpir's How to use Objectives and init.sqs.
Oh, and if you want my advice, switch to playing in Veteran mode. Cadet mode is occasionally useful for beta testing but that's about all its good for. It negates the game's greatest strength - it's openness.