Copy the video below.

If your using an older version of Unreal the Setting Behaviour and Player Mappable Key Settings options in the SGKCharacter Inputs will be different, just tick on the IsPlayerMappable option and set the Player Mappable Options the same as the video below.

Next open the BP_PlayerInventory and create a new Boolean variable called QuickDropKeyDown

Next add the highlighted nodes shown in the images below, locations at the tops of the images.

The default key bind is Left Alt, this must be held down when clicking the item in the inventory to quick drop it.