Vent tube on Gas Tank Sending Unit?

I’ve got a 65 Barracuda. Turns out when we put the sending unit in we never put anything on the nipple which I believe is some sort of vent? How do I set this up? Is it a rubber hose or is it a metal tube and where does it go? And how do I get one?

FF477321-B0D7-4EA0-8B2D-E005C7C1E1E7.png