Getting the entity a player is viewing the GUI of.

So basically, i want players to be able to put armor on my custom entity through a GUI. However, by using the "event/target" block it refers to me instead of the entity i'm checking the GUI of. The armor is still left in it's GUI. I believe the items is saved locally per the entities GUIs but i cant find the armor item while trying to find it through their inventories.

So i want the armor to behave exactly like the vanilla horses do. When the armor is placed in it's inventory/GUI, it will change into its armored texture (hopefully).

Have been searching for a while on how to do this with no luck :/, so anything helps.

Thanks in advance.