Application.php

<?php

declare(strict_types=1);

namespace OCA\SendWelcomeMail\AppInfo;

use OCP\AppFramework\App;
use OCP\User\Events\UserCreatedEvent;
use OCA\SendWelcomeMail\Listener\UserCreatedEventListener;
use OCP\EventDispatcher\IEventDispatcher;


class Application extends App
{
  public function __construct()
  {
    parent::__construct('sendwelcomemail');
    $container = $this->getContainer();
    /* @var IEventDispatcher $eventDispatcher */
    $dispatcher = $container->query(IEventDispatcher::class);
    $dispatcher->addServiceListener(UserCreatedEvent::class, UserCreatedEventListener::class);
  }
}

UserCreatedEventListener.php

<?php

namespace OCA\SendWelcomeMail\Listener;

use OCP\User\Events\UserCreatedEvent;
use OCP\EventDispatcher\Event;
use OCP\EventDispatcher\IEventListener;

class UserCreatedEventListener implements IEventListener
{

  public function __construct()
  {
    echo("HELLO TEST");
    $this->logger->error("HELLO SERVICE!");
    throw new \Exception("HELLO SERVICE");
  }

  public function handle(Event $event): void
  {
    if (!($event instanceof UserCreatedEvent)) {
      return;
    }
    $this->logger->error("HELLO SERVICE!");
  }
}