[Crow]

You probably know of the onPlayerChats event. You should check if the player's chat begins with your desired string there ("//" in this case) and then send a trigger to the serverside. There, you can use sendToRC() or echo() to send a message to RC. If you're using echo(), only people with NPC-Control will be able to see it (unless sendechotorc is set to true in the server options).