Hi, Jay here from the OneNote Integrations team.
Did you know that the OneNote desktop version on Windows has an interface for extensibility? Popular utilities like Onetastic were implemented using this interface. Using this, you can develop and install your own COM add-ins which would then show up on the OneNote ribbon.
There are a few samples and sites out there out there describing how to create such an add-in, but oftentimes there are many error-prone steps involved. To help people get started creating such add-ins, we've created a sample to help folks get bootstrapped. You can find it on GitHub here.
In short, you just need to take the sample, rename a couple of files, and replace a few strings, and then you're ready to go :).
This is still a work in progress, feel free to let us know of any bugs (or do a pull request 🙂 ).