ecc_cludo_search.module

<?php

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function ecc_cludo_search_form_cludo_search_search_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $form['basic']['search_keys']['#title'] = t('Enter the terms you wish to search for');
  $form['basic']['search_keys']['#title_display'] = 'invisible';
}

/**
 * Implements hook_form_FORM_ID_alter().
 */
function ecc_cludo_search_form_cludo_search_block_search_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $placeholder = t('Search @site.name', [
    '@site.name' => \Drupal::config('system.site')->get('name')]
  );
  $form['search_keys']['#attributes']['placeholder'] = $placeholder;
  $form['search_keys']['#attributes']['onfocus'] = "this.placeholder = ''";
  $form['search_keys']['#attributes']['onblur'] = "this.placeholder = '" . $placeholder . "'";
}