Cognito Post Confirmation trigger

Question

var params = {
  policyName: 'STRING_VALUE', /* required */
  target: 'STRING_VALUE' /* required */
};
iot.attachPolicy(params, function(err, data) {
  if (err) console.log(err, err.stack); // an error occurred
  else     console.log(data);           // successful response
});

I need to use above Method in a Lambda function to attach cognitoIdentityId with policyName https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/Iot.html#attachPolicy-property. I have attached Cognito Post Confirmation trigger with a lambda function and getting event successfully. https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-post-confirmation.html I am looking for a way to get cognitoIdentityId using this event. Is there an API for this purpose or workaround to achieve this.