gistfile1.txt

<?php

/**
 * Form alter that just disables the preview button.
 */
function caxy_form_contact_message_feedback_form_alter(&$form, FormStateInterface $form_state) {
  $form['actions']['preview']['#access'] = FALSE;
}

/**
 * Form alter that disables the preview button and replaces a value of a form element.
 */
function caxy_form_contact_message_job_posting_form_form_alter(&$form, FormStateInterface $form_state) {
  if (Drupal::request()->attributes->has('node')) {
    /** @var \Drupal\node\Entity\Node $node */
    $node = Drupal::request()->attributes->get('node');
    $form['subject'] = array(
      '#type' => 'value',
      '#value' => $node->getTitle(),
    );
  }
  $form['actions']['preview']['#access'] = FALSE;
}