The owner's manual refers to it as a port flush cap, but the parts breakdown refers to what it is.... a grommet. It is just a rubber plug to plug the hole.
It can be found in the "extension case" section of the parts breakdown. As you can see from the parts breakdown, the dotted line infers that the grommet goes into the hole in the lower cover.
As I said before, it will not necessarily harm the engine to run without it. Your hole is already open now with the flush attachment connected. Any opening there just allows debris and water to get under the hood. It is always good to keep that to a minimum but not catastrophic.
Mike
Mike