Keyboards Keys ('/')?

The slash key doesn't trigger the KeyDown event, but it does trigger the KeyUp event. An example of how to do this is below.

local Mouse = game.Players.LocalPlayer:GetMouse()

Mouse.KeyUp:connect(function(Key)
	if Key == "/" then
		print("The slash key was used")
	end
end)

However, you should really use UserInputService, as using KeyUp means there would be a short delay between when they pressed '/' and when they could type. If you did use UIS, you would use the InputBegan event and read the KeyCode property of the given InputObject.

local UIS = game:GetService("UserInputService")

UIS.InputBegan:connect(function(Input)
	if Input.KeyCode == Enum.KeyCode.Slash then
		print("The slash key was pressed")
	end
end)