Sorry, for some reason I got you confused with someone that has a K75S that I've been corresponding with. Pre 1986 K100s are weird animals and have all kinds of unique items that I'm not fully familiar with. That tank vent might be one of them. Hopefully someone with specific '85 knowledge will weigh in.
If that is the tank vent, you will need to connect a hose to it and run it down to behind the right peg plate so it vents to the ground. Check both tubes with air to see which is the tank vent and which is the rain drain. Sorry for the mixup.