Register a guest account on this HS.
        Note: HS must have guest registration enabled.
        Returns:
            str: Access Token
        Raises:
            MatrixRequestError

def register_as_guest(self):
        """ Register a guest account on this HS.
        Note: HS must have guest registration enabled.
        Returns:
            str: Access Token
        Raises:
            MatrixRequestError
        """
        response = self.api.register(auth_body=None, kind='guest')
        return self._post_registration(response)