Backend Plugins
We currently don't host a backend API. If you were hoping to create a plugin which makes use of the backend API, we suggest that instead you try to build it via the frontend API.
For example, instead of writing a program which sends data to RemNote from a third party service, use a frontend plugin to request data from the third party service each time RemNote loads and import the data.
For a real life example of this, see the Readwise plugin which used to use the backend API, but has been rebuilt as a frontend plugin.