Is there a way to get the value of a player's selected inventory slot in

An Integer that specifies the position of a positioned item stack Equipment Slots Item Stack Slot, item Slot, Inventory NBT Slot 40

A slot is a location in the data of a block entity or entity that an item stack can be placed in A slot is defined by its slot index

minecraft slot Now if for some reason you truly need to store the slot, you could do this by using slot ids, as long as all of the slots don't change names (

minecraft slot machine How many inventory slots are in Minecraft? Delve into the specifics of Minecraft's inventory system, including hotbar and storage slots