Steering head cap, look under the bars
Yup, there are 2 of them.
Luckily, It doesn't look like it's needs to "seal" anything in.
2016 F6B Standard Matte Silver. Freedom Combat Slip-Ons, BaggerShield 11" - 19" Sport Shield, Kuryakyn Ergo II Highway Pegs, Ultimate King Seat w/Driver Backrest, Pathfinder Running/Turnsignal/Driving Lights, Beeline Moto Navigation, INNOVV K2 Camera, DHS Racing Shifter Linkage Upgrade, Show Chrome Handlebar Risers and Luggage Rack.
I have mbl risers so maybe under that? but I do not see anywhere else in the area that would hold that plug. I'll save it, have a drawer full of items like this over the years.
2013 F6B 1800B
Don't be a thermometer, be a thermostat
Most likely the cap that fits over the bolt holding the riser against the triple tree.
Prevents water collection in the depression.
Check inside the bags where they secure to the frame. Each bag has 2 of those caps covering a bolt and keeping moisture out.