code: public function load(ObjectManager $manager)
      {
          $userGroup = $this->getReference('user-group');
          $adminGroup = $this->getReference('admin-group');
  
          $user = new User();
          $user->setFirstName('Administrative');
          $user->setLastName('User');
          $user->setUsername('admin');
          $user->setPassword('password');
          $user->addGroup($userGroup);
          $user->addGroup($adminGroup);
  
          $factory = $this->container->get('security.encoder_factory');
          $encoder = $factory->getEncoder($user);
          $user->setPassword($encoder->encodePassword($user->getPassword(), $user->getSalt()));
  
          $this->setReference('admin-user', $user);
  
          $manager->persist($user);
  
          $manager->flush();
      }
docs: Load data fixtures with the passed EntityManager
  
  @param \Doctrine\Common\Persistence\ObjectManager $manager