I want to put starting inventory in a second inventory GUI. But I don't know how to add Items to the second Inventory box only the first.
Each InventoryWindow has a character it belongs to, get-/settable via the .CharacterToUse property.
Use a dummy character and add the items to their inventory.
There are a few more bumps down the road, but since this question has come up before, the thing you should have down before everything else is use the forum search.