contact_form_user_admin_settings_submit

contact_form_user_admin_settings_submit($form, FormStateInterface $form_state)

function contact_form_user_admin_settings_submit($form, FormStateInterface $form_state) {
  \Drupal::configFactory()->getEditable('contact.settings')
    ->set('user_default_enabled', $form_state->getValue('contact_default_status'))
    ->save();
}