docs: def register(self, username=""):
          """Performs /register with type: m.login.application_service
  
          Args:
              username(str): Username to register.
          """
          if not username:
              username = utils.mxid2localpart(self.identity)
          content = {
              "type": "m.login.application_service",
              "username": username,
          }
          return self._send("POST", "/register", content,
                            api_path=MATRIX_V2_API_PATH)