There is something missing for sure, but I don't think anything was broken.
The missing piece probably wasn't included with the bike when sold because because of the trim package that it included. The parts fiche seem to be silent on the issue.
The fiche seems to be screaming up here. The hose-covered peg goes in that hole.To which missing piece are you referring—the one that seems broken off to me or a different one?
Here's how I see it. The threaded portion of the stud you bought is inserted into the horseshoe molding on the rear of the cover. The stud's smooth part faces to the rear. A washer is placed on the threaded part then the nut is tightened onto the washer. An imprint of a washer is visible on the interior face of the horseshoe. A piece of hose of the type indicated in the fiche is slipped over the smooth length of stud then the assembly is inserted into the hole. I don't how you'll deal with the missing piece.
If I stare at it long enough I can almost see my description working, but not quite.

The contour of the cover's aft mount seems designed to snap the cover post into place. If the part were a couple of thousand miles closer, I'd be interested to take a whack at fitting it. Speaking of whack, the fiche is selling a meter of submersible petrol hose for the application.

At least that gives hint of the finished post's diameter, and extra fuel hose is always useful.