Drupal 8 add message

 \Drupal::messenger()->addMessage($message, 'status');
 \Drupal::messenger()->addMessage($message, 'warning');
 \Drupal::messenger()->addMessage($message, 'error');

Source: api.drupal.org

Drupal set message

$messenger = \Drupal::messenger();
$messenger->addMessage('STUFF', $messenger::TYPE_WARNING);

Source: api.drupal.org

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