I am trying to create the Redirect to URL after clicking email verification link (explained in this github issue). I am also following these Stackoverflow replies

I have a Cognito user pool with a single Lambda trigger: Custom message Lambda trigger, which looks like this:

exports.handler = (event, context, callback) => {
    console.log("event")
    console.log(event)
    // Identify why was this function invoked
    if(event.triggerSource === "CustomMessage_SignUp") {
        console.log("editing email message...")
        const { codeParameter } = event.request
        const { userName, region } = event
        const { clientId } = event.callerContext
        const { email } = event.request.userAttributes
        const url = 'https://my-api.execute-api.us-east-1.amazonaws.com/dev/redirect'
        const link = `<a href="${url}?code=${codeParameter}&username=${userName}&clientId=${clientId}&region=${region}&email=${email}" target="_blank">here</a>`
        console.log(link)
        event.response.emailSubject = "Your verification link"; // event.request.codeParameter
        event.response.emailMessage = `Thank you for signing up. Click ${link} to verify your email.`;
    } else {
        console.log("trigger source was invalid")
    }

    // Return to Amazon Cognito
    callback(null, event);
};

Then I made an API Gateway with a dev stage, a redirect resource and a GET method pointing to a confirm_user Lambda as explained in this Medium tutorial. The Lambda looks like this:

'use strict';
var AWS = require('aws-sdk');
AWS.config.setPromisesDependency(require('bluebird'));
var CognitoIdentityServiceProvider = new AWS.CognitoIdentityServiceProvider({ apiVersion: '2019-11-07', region: 'us-east-1'});

exports.handler = (req, context, callback) => {

  console.log('req');
  console.log(req);
  const confirmationCode = req.code
  const username = req.username
  const clientId = req.clientId
  const region = req.region
  const email = req.email

  let params = {
    ClientId: clientId,
    ConfirmationCode: confirmationCode,
    Username: username
  }

  var confirmSignUp = CognitoIdentityServiceProvider.confirmSignUp(params).promise();

  confirmSignUp.then(
    (data) => {
      let redirectUrl = process.env.POST_REGISTRATION_VERIFICATION_REDIRECT_URL;
      const response = {
        statusCode: 302,
        headers: {
          Location: redirectUrl,
        }
      };

      return callback(null, response);
    }
  ).catch(
    (error) => {
      callback(error.message)
    }
  )
}

In Cognito -> Messaging -> Verification message, I put verification type: code.

This is how my user pool sign-up experience looks like:

My expectation is that Cognito generates the custom message, the user gets it in the email. When they click on it, they get confirmed and redirected to my website.

What happens is that Cognito generates the custom message, then for some seconds nothing happens, then the API gets called, the confirm_user Lambda gets called, the user gets confirmed, and then the email arrives. When clicking the link the code is expired because it was already used.

I cannot figure out why the link is being called in the background, which makes the user get confirmed even before they get the email.

Any help is appreciated!