Leaking out of hole

Question

Hi

I went to fill my Hoover Steam Vac and the hole is open at bottom. I went to machine and a black post with a spring on it (reminds me of inside on a pen) was just lying there. I tried to figure out how to attach it back, but I wonder if another part is missing. There is a rubber gasket on the base that it fits onto, but it doen't remove.