Widgets are only rendered on the page itself right now https://github.com/NodeBB/NodeBB/blob/master/src/middleware/render.js#L53. We would have to add support so they are rendered in renderHeader method as well. Open an issue on our github it could be a nice addition.
Hello, I created a widget that adds to the forum displaying active users on your discord!
Just put this code in the LESS / CSS tab and in the widgets!
Everything looks like this