mailjet-8.x-2.7/modules/mailjet_stats/src/EventSubscriber/InitSubscriber.php

<?php

/**
 * @file
 * Contains \Drupal\mailjet_stats\EventSubscriber\InitSubscriber.
 */

namespace Drupal\mailjet_stats\EventSubscriber;

use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class InitSubscriber implements EventSubscriberInterface
{

  /**
   * {@inheritdoc}
   */
    public static function getSubscribedEvents(): array
    {
        return [KernelEvents::REQUEST => ['onEvent', 0]];
    }

    public function onEvent()
    {
        if (isset($_GET['token'])) {
            $mailjet_campaign_id = $_GET['token'];
            $_SESSION['mailjet_campaign_id'] = $mailjet_campaign_id;
        }
    }

}