How to set a placeholder to a form element in a exposed view filter?
You can implement the hook_form_FORM_ID_alter() hook, in the case of the view exposed filter form the name is views_exposed_form

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function MODULE_form_views_exposed_form_alter(array &amp;$form, FormStateInterface $form_state, $form_id) {
  //You need to verify the id
  switch ($form['#id']) {
    case 'views-exposed-form-nouvelles-page-1':
      //Setting placeholder
      $form['title']['#attributes']['placeholder'] = $form['#info']['filter-title']['label'];
      //Cleaning the filter labels
      unset($form['#info']['filter-title']['label']);
      break;
  }
}


How you can find the element name? It's easy, you can Inspect your Element: