code: def postWebhook(self, dev_id, external_id, url, event_types):
          
          path = 'notification/webhook'
          payload = {'device': {'id': dev_id}, 'externalId': external_id,
                     'url': url, 'eventTypes': event_types}
          return self.rachio.post(path, payload)
docs: Add a webhook to a device.
  
          externalId can be used as opaque data that
          is tied to your company, and passed back in each webhook event
          response.