Even though the IPL shows a hose that goes from that barb to a primer, that system doesn't exist at this time. There is no hose or primer that is supposed to be connected to that barb on the carb. The primer located on the air filter housing pushes air into the hole next to the large carb entrance hole. The barb has to be present for the primer to work but nothing is connected to that barb.
Actually the same question came up around 2015 when this style engine with this carb kit the market. I think the barb is there and they list a part number for the hose for some future project that includes a remote primer, but as of right now doesn't exist.