code: def _make_signing_request(self, message):
          
          message = _helpers.to_bytes(message)
  
          method = 'POST'
          url = _SIGN_BLOB_URI.format(self._service_account_email)
          headers = {}
          body = json.dumps({
              'bytesToSign': base64.b64encode(message).decode('utf-8'),
          })
  
          self._credentials.before_request(self._request, method, url, headers)
          response = self._request(
              url=url, method=method, body=body, headers=headers)
  
          if response.status != http_client.OK:
              raise exceptions.TransportError(
                  'Error calling the IAM signBytes API: {}'.format(
                      response.data))
  
          return json.loads(response.data.decode('utf-8'))
docs: Makes a request to the API signBlob API.