contact_form_user_form_alter

contact_form_user_form_alter(&$form, FormStateInterface $form_state)

function contact_form_user_form_alter(&$form, FormStateInterface $form_state) {
  $form['contact'] = array(
    '#type' => 'details',
    '#title' => t('Contact settings'),
    '#open' => TRUE,
    '#weight' => 5,
  );
  $account = $form_state->getFormObject()->getEntity();
  if (!\Drupal::currentUser()->isAnonymous() && $account->id()) {
    $account_data = \Drupal::service('user.data')->get('contact', $account->id(), 'enabled');
  }
  $form['contact']['contact'] = array(
    '#type' => 'checkbox',
    '#title' => t('Personal contact form'),
    '#default_value' => isset($account_data) ? $account_data : \Drupal::config('contact.settings')->get('user_default_enabled'),
    '#description' => t('Allow other users to contact you via a personal contact form which keeps your email address hidden. Note that some privileged users such as site administrators are still able to contact you even if you choose to disable this feature.'),
  );
  $form['actions']['submit']['#submit'][] = 'contact_user_profile_form_submit';
}