EquipItemByLink

This is a user-defined function that you can copy and paste into your addon.

Equips the first matching item found in the player's bags (including the bank and bank bags).
Checks the bank first, then carry-ons, then bank bags.

local function EquipItemByLink(link)
	for bag=BANK_CONTAINER, NUM_BAG_SLOTS+NUM_BANKBAGSLOTS do
		for slot=1,GetContainerNumSlots(bag) do
			local item = GetContainerItemLink(bag, slot)
			if item and item == link then
				if CursorHasItem() or CursorHasMoney() or CursorHasSpell() then ClearCursor() end
				PickupContainerItem(bag, slot)
				AutoEquipCursorItem()
				return true
			end
		end
	end
end