Drupal 8: Redirect to page from Event Subscriber with destination query

Create your Event Subscriber

Create your module's services yml file and declare the event subscriber
```
services:
  mymodule.autologin:
    class: Drupal\mymodule\EventSubscriber\AutoLogin
    tags:
      - {name: event_subscriber}
```
Save this as mymodule.services.yml inside mymodule folder

Create the actual class used from above code

<?php

namespace Drupal\mymodule\EventSubscriber;

use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class AutoLogin implements EventSubscriberInterface {
  public function redirectToLogin() {
     // PUT YOUR LOGIC HERE
  }

  /**
    * {@inheritdoc}
    */
  public static function getSubscribedEvents() {
    $events[KernelEvents::REQUEST][] = ['redirectToLogin'];
    return $events;
  }
}

Save this as AutoLogin.php inside mymodule/src/EventSubscriber folder

Redirect using URL::fromRoute with destination query

$current_path = \Drupal::service('path.current')->getPath();

$url = \Drupal\Core\Url::fromRoute('simplesamlphp_auth.saml_login', [], ['query' => ['destination' => $current_path, 'absolute' => TRUE]]);

$response = new RedirectResponse($url->toString(), 302);
$event->setResponse($response);

Put the above code in the AutoLogin::redirectLogin().

This code will get the current path and redirect to saml login page to auto login, then if the login is successful, it will redirect to the original path that we are trying to access.