RainLab\User\Controllers\Users::formExtendFields PHP Method

    public function formExtendFields($form)
    {
        /*
         * Show the username field if it is configured for use
         */
        if (UserSettings::get('login_attribute') == UserSettings::LOGIN_USERNAME && array_key_exists('username', $form->getFields())) {
            $form->getField('username')->hidden = false;
        }
    }