mines 2017 an defiantly an allen key
Just measure it across the flats... with an imperial measure. It looks like it is 1/4" hex driver you need.
If you are going to be working on the bike further then you are going to need other hex drivers. It would be worth looking over the bike to see but a set for occasional use doesn't have to be expensive.
its defiantly an allen key 6mm fits but not as good as id like i maybe will phone the dealer an ask them
It won't be a metric size. Its Imperial .
If it's any help, my old dyna and my current softail are 1/4 inch, just make sure it's a real snug fit and don't be tempted to use the ball end.
Round them off and your in a world of poo. Good luck.
If you acquire a set of A/F Allen keys one of them is bound to fit.