So what is this cap and what is its function?
Looks like mine is missing on my '12 but I'm having no runability issues.
The tube is the vacuum source for the evaporative emissions system purge valve, and it is capped off on units that do not have that system, such as 49 state models like yours. Get a rubber cap, clean the tube with a solvent like alcohol so the cap won't be easily dislodged, and install the cap to the tube. There are adhesives that can be used to make sure the cap doesn't come off again if you think that might be a problem.
You can buy a rubber cap from your friendly dealer, or just go to any auto parts store and buy the small assortment pack that has several sizes and chose the best snug fit. The Harley part number is 90383-98.
Jerry