Help Item From Inventory To Array

Hello, i made an npc (instance) that player can store an item from his inventory to an array and this item will be appear in the npc

The question is how i can make the player choose item from inventory. So i want make a npc that when u press "add"

it will open a window like normal trade and you choose 2-3 items and press ok and thsoe will be added in Array_ X

e.t.c

how to do this trade?