code: def _url_for(endpoint: str, **values) -> Union[str, None]:
      
      _external_host = values.pop('_external_host', None)
      is_external = bool(_external_host or values.get('_external'))
      external_host = _external_host or current_app.config.get('EXTERNAL_SERVER_NAME')
      if not is_external or not external_host:
          return flask_url_for(endpoint, **values)
  
      if '://' not in external_host:
          external_host = f'http://{external_host}'
      values.pop('_external')
      return external_host.rstrip('/') + flask_url_for(endpoint, **values)
docs: The same as flask's url_for, except this also supports building external
      urls for hosts that are different from app.config.SERVER_NAME. One case
      where this is especially useful is for single page apps, where the frontend
      is not hosted by the same server as the backend, but the backend still needs
      to generate urls to frontend routes
  
      :param endpoint: the name of the endpoint
      :param values: the variable arguments of the URL rule
      :return: a url path, or None