How to verify reCAPTCHA with the help of Total.js?

NEWSCHEMA('SignUp').make(function(schema) {
    
    // ...
    schema.define('email', 'Email', true);
    schema.define('recaptcha', 'String', true);
    // ...

    schema.addWorkflow('reCAPTCHA', function(error, model, options, callback, controller) {
        RESTBuilder.make(function(builder) {
            builder.url('https://www.google.com/recaptcha/api/siteverify');
            builder.set('secret', 'YOUR SECRET FROM GOOGLE');
            builder.set('response', model.recaptcha);
            controller && builder.set('remoteip', controller.ip);
            builder.urlencoded();
            builder.exec(function(err, response) {
                if (err || !response.success)
                    error.push('error-recaptcha');
                callback(SUCCESS(true));
            });
        }); 
    });

    schema.addWorkflow('notify', function(error, model, options, callback, controller) {
        // ...
    });

    schema.setSave(function(error, model, options, callback, controller) {
        // ...
    });

});

exports.install = function() {
    F.route('/api/signup/', json_signup, ['*SignUp', 'post']);
};

function json_signup() {
    var self = this;
    self.$async(self.callback(), 1).$workflow('recaptcha').$save().$workflow('notify');
}