You need to put index.php at root
$uid ='Your User ID Here';
$account = \Drupal\user\Entity\User::load($uid);
\Drupal::service('session')->migrate();
\Drupal::service('session')->set('uid', $account->id());
\Drupal::moduleHandler()->invokeAll('user_login', array($account));
or
$uid ='Your User ID Here';
$user = Drupal\user\Entity\User::load($uid);
user_login_finalize($user);
$response->send();