How to change search placeholder?

So far I have tried this code in mytheme.theme:

function mytheme_form_search_block_form_alter(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id) {
    $form['keys']['#attributes']['placeholder'] = t('Custom wording');
}

And this one:

function mytheme_form_alter(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id) {
  if ($form_id == 'search_block_form') { 
    $form['keys']['#attributes']['placeholder'] = t('Custom wording');
  }
}

Also this one:

function mytheme_form_search_block_form_alter(&$form, $form_state) {
    $form['keys']['#attributes']['placeholder'] = t('Custom wording');
}

Site is not crashing, but nothing happens on the placeholder.

I have based these 3 snippets on these pages:

• https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_FORM_ID_alter/8.5.x

• How to replace the search submit button with a Font Awesome character?

function mytheme_form_search_block_form_alter(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id) {
  $form['keys']['#attributes']['placeholder'] = t('The new placeholder');
}