Create watch as notification for response in Google Forms

Hi!

I'm trying to reproduce a sample in AppsScript that has been published in the Google Documentation for python and Node.
It is this method: https://developers.google.com/forms/api/guides/push-notifications#create_a_watch

So basically, I have a topic created in the Cloud Pub/Sub feature for my project and want to have some kind of notification process as when a new response of Google Form has been submitted. I cannot use an email notification but I'm open to any other solution that provides me the functionality of triggering a subprocess of a Node app.

Here's the use case: For MVP reasons and getting quick results, I have embedded the form as an iframe in my Node app. When users complete the form I need to start a new process with the information provided in the form as the input.
For the time being, I thought about having set up an AppsScript in the form that publishes to a Pub/Sub topic to which the app is subscribed.

However, using the following code:

 function createWatch() {
   Logger.log('Calling the Forms API!');
   var formId = FORMID';

   // Get OAuth Token
  var OAuthToken = ScriptApp.getOAuthToken();
  Logger.log('OAuth token is: ' + OAuthToken);
  var formsAPIUrl = 'https://forms.googleapis.com/v1beta/forms/' + formId + '/' + 'watches';
  Logger.log('formsAPIUrl is: ' + formsAPIUrl);
  var watchRequest = JSON.stringify({
    "watch": {
        "target": {
          "topic": {
            "topicName": "projects/xxx/topics/xxx"
          }
        },
        "eventType": "RESPONSES"
      }
    });
  var options = {
     'headers': {
       Authorization: 'Bearer ' + OAuthToken,
       Accept: 'application/json'
     },
     'method': 'post',
     'muteHttpExceptions': true,
     'payload': watchRequest
   };  

  var response = UrlFetchApp.fetch(formsAPIUrl, options);
  Logger.log('Response from forms.responses was: ' + response);
}

... I get the following response:

{
  "error": {
    "code": 400,
    "message": "Invalid JSON payload received. Unknown name \"{\"watch\":{\"target\":{\"topic\":{\"topicName\":\"projects/fachmarkt-manager/topics/employee-onboarding\"}},\"eventType\":\"RESPONSES\"}}\": Cannot bind query parameter. Field '{\"watch\":{\"target\":{\"topic\":{\"topicName\":\"projects/fachmarkt-manager/topics/employee-onboarding\"}},\"eventType\":\"RESPONSES\"}}' could not be found in request message.",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.BadRequest",
        "fieldViolations": [
          {
            "description": "Invalid JSON payload received. Unknown name \"{\"watch\":{\"target\":{\"topic\":{\"topicName\":\"projects/fachmarkt-manager/topics/employee-onboarding\"}},\"eventType\":\"RESPONSES\"}}\": Cannot bind query parameter. Field '{\"watch\":{\"target\":{\"topic\":{\"topicName\":\"projects/fachmarkt-manager/topics/employee-onboarding\"}},\"eventType\":\"RESPONSES\"}}' could not be found in request message."
          }
        ]
      }
    ]
  }
}

I have also tried to drop the stringify of the payload but it results in the same error.

Any chances of fixing this?