Help with a Procedure

That make more than 2Day that I try make a tracker where we can track player. 
I explain, 
I have the item Tracker but don't have a procedure for save player in, and if player is aleadry saved that give to the player who click with the tracker the cordonate of the traced player.
I'm trying to make that for more than 2day but without any succes if someone can help me that could be very nice ! :D 

---=Information=---
McReator version : 2020.5
Forge version : 1.12.2 (with the plugin :)

Item name : Tracker

Thanks you very much to every poeple who will answer <3 ​​​​​​​