function validateRecaptcha(callback) {
    if (useRecaptcha) {
        grecaptcha.ready(function () {
            grecaptcha.execute(GOOGLE_RECAPTCHA_SITE_KEY, {action: 'submit'}).then(function (recaptchaToken) {
                callback(recaptchaToken);
            });
        });
    } else {
        callback(null);
    }
}