If I am understanding correctly you want to spawn with the weapon attached to your character. If so, you would have to use the Event BeginPlay. Make sure that you drag and drop the actor/weapon into a random unreachable location so that begin play gets called when you start the level.
Before I used the key 4 to add a weapon to my players hand. Now I got an inventory, where the Event “Use item” is called, when I press equip on the weapon in the inventory. The event is called, I checked it with a print, but the weapon doesnt get equipped to the players hand. That is my problem.