Customization in drupal 8 core contact form using form alter

How to change Drupal 8 Core Contact form fleld label :-

 If you want to change Contact form "Message" field label you can use "hook_form_alter" .

either you can create a custom module or either you can add form alter in your theme file also.

 

function your_theme_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
       if ($form_id == 'contact_message_feedback_form') {
        $form['message']['widget'][0]['value']['#title'] = t("My Custom Body Label");
     }
} 

Add placeholder in default search using form alter

function your_theme_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
    if (in_array($form_id, ['search_block_form', 'search_form'])) {
        $form['keys']['#attributes']['placeholder'] = t('Search...');
    }
   }if ($form_id == 'contact_message_feedback_form') {
  $form['actions']['submit']['#value'] = t("My Altered submit button text here");
  $form['subject']['widget'][0]['value']['#title'] = t ("My Altered title here");
  $form['message']['widget'][0]['value']['#title'] = t("My custom body text here");
        //$form['message']['widget'][0]['value']['#title'] = t("My Custom Body Label");
  $form['#suffix'] = "<div class='extra-text'><p>Show some text before the comment form.</p></div>";
     }

}