How can I remove preview button from contact form? How to turn off preview in contact form in Drupal 8.1.x
A custom module can utilize hook_form_alter() to strip out the preview button form element in any form:

/**
 * Implements hook_form_alter().
 */
function MYMODULE_form_alter(&amp;$form, $form_state, $form_id) {

  // Look for any form provided by the contact module.
  // If you want to target a specific form you'll use the whole form ID
  // (e.g. Website feedback = 'contact_message_feedback_form').
  if (strpos($form_id, 'contact_message_') !== FALSE) {
    $form['actions']['preview']['#access'] = FALSE;
  }
}