The skillbar, like any other window containing items, is a collection.
Key combinations can be set up using the keys field. Any number of fields can be used, and the skillbar's slot will auto. be generated based on the amount of keys you have defined. When using references multiple wrappers can be used to define it's behavior.
For example, when using the UI_Item_KeyTriggerer_PFB (as shown in the picture) stacks will behave as normal and will deplete as the item referenced is depleted. The UI_Item_reference_Sum_PFB wrapper creates a sum (total amount of set item in the player's inventories) and shows this as the actual value.
Using a wrapper that implements the IInventoryItemWrapperKeyTriggerer interface is required.