My son lost the right inner fender on his car and now I see a white tank. It is empty I know the one on the left side (drivers side) is for window washing fluid. What does this one do?

It passes to the engine compartment through a black rubber gromment but appears to be open. In other words, the white plastic tank has a neck that presses up against the black rubber but doesn/t extend pass it.

When I look from the engine side it looks like a oval cut out with nothing covering the opening.