How to make Bedrock edition "loot crates"

DanRobzProbz made a factions world download on Youtube. It sadly broke, because of the village and pillage update broke the villagers.

There is "loot crates" that you click a shulker box with a "loot key." How would I make my own loot key system just with minecraft’s addon system? I know how to make addons (but I am not good at it).