cohesion_theme.theme

<?php

/**
 * @file
 */

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_alter().
 */
function cohesion_theme_form_alter(&$form, FormStateInterface $form_state, $form_id) {

  // Add placeholder to search block form input.
  if ($form_id === 'search_block_form') {
    $form['keys']['#attributes']['placeholder'] = t('Search');
  }
}