contact_form_user_admin_settings_alter

contact_form_user_admin_settings_alter(&$form, FormStateInterface $form_state)

function contact_form_user_admin_settings_alter(&$form, FormStateInterface $form_state) {
  $form['contact'] = array(
    '#type' => 'details',
    '#title' => t('Contact settings'),
    '#open' => TRUE,
    '#weight' => 0,
  );
  $form['contact']['contact_default_status'] = array(
    '#type' => 'checkbox',
    '#title' => t('Enable the personal contact form by default for new users'),
    '#description' => t('Changing this setting will not affect existing users.'),
    '#default_value' => \Drupal::configFactory()->getEditable('contact.settings')->get('user_default_enabled'),
  );
  // Add submit handler to save contact configuration.
  $form['#submit'][] = 'contact_form_user_admin_settings_submit';
}