Custom Routes
NodeCG uses express for its routing, and exposes a helper for creating routes easily:
bundles/my-bundle/extension.js
module.exports = function(nodecg) {
    const router = nodecg.Router();
    router.get('/customroute', (req, res) => {
        res.send('OK!');
    });
    nodecg.mount('/my-bundle', router); // The route '/my-bundle/customroute` is now available
};
NodeCG has a Login Security Middleware that is by default not enabled. To enable it, you need to add it. This won't work unless Security & Authentication is enabled.
bundles/my-bundle/extension.js
module.exports = function(nodecg) {
    const router = nodecg.Router();
    router.use(nodecg.util.authCheck); // All routes using "router" will need a `?key=` in it's URL to be avaliable 
    router.get('/customroute', (req, res) => {
        res.send('OK!');
    });
    nodecg.mount('/my-bundle', router); // The route '/my-bundle/customroute` is now available
};