how to create user-defined setting in jupyter_notebook_config.json and read it

Question

I create a user-defined setting in jupyter_notebook_config.json, then I want to read it in my jupyter server extension, but I got a None value.

here is my setting file:

{
  "NotebookApp": {
    "nbserver_extensions": {
      "auth_ext": true
    },
    "info_url": "http://127.0.0.1:7000/info"
  }
}

and then I read it:

class RouteHandler(APIHandler, ABC):
    @tornado.web.authenticated
    def get(self):
        info_url = self.settings.get("info_url")
        self.log.info("info_url: %s" % info_url)