function MYMODULE_preprocess_node(&$variables) {
 $node = $variables['elements']['#node'];
  $flag_link_service = Drupal::service('flag.link_builder');
$flag_link = $flag_link_service->build($node->getEntityTypeId(), $node->id(), 'YOUR_FLAG_MACHINE_NAME');
$variables['flag_link'] = $flag_link;
}

If ($user->uid) == 0)
 {
redirect to login
 {
 else
 {
   {{ flag_link }}
   }