mailjet-8.x-2.7/modules/mailjet_trigger_examples/src/Controller/TriggerExamplesController.php

<?php

/**
 * @file
 * Contains
 *   \Drupal\mailjet_trigger_examples\Controller\TriggerExamplesController.
 */

namespace Drupal\mailjet_trigger_examples\Controller;

use Drupal\Core\Entity\ContentEntityBase;
use Drupal\user\Entity\User;
use Drupal\message\Entity\Message;
use Drupal\message\Entity\MessageTemplate;
use Drupal\Core\Controller\ControllerBase;

class TriggerExamplesController extends ControllerBase
{

    public function content()
    {

        global $base_url;
        return mailjetExternalLinkRedirect($base_url . '/admin/structure/message');
    }

}