Help with identifying players

So I would like to make my code regen the player and place an arrow in a specific place when that player gets hurt. But I am using a plugin that makes me allow to make spigot plugins and it does not allow me to do that global trigger. So how would I do it? I tried just saying: "If event/target entity health gets lower than 20 then send a message" just to test but it did not work, I'm guessing because I did not define "event/target entity"