Go to the tab General => Navigation, drag the green square on the right with Custom Route next to it over to the navbar looking area near the top. Then click the new Icon that appears and fill out the fields, put the url in the route field. Then click Enable and then click the Save Icon at the bottom right.
It must be better to fix the requested requirement instead of migrating out of NodeBB. I ❤ this projekt and my goal is to learn more about its architecture. One problem, time ... But I will get there 😛