Recurly.buildSubscriptionForm({
    target: '#recurly-subscribe',
    afterSubscribe: function(result) {
      var data = {
        subscription: result
      , somethingExtra: $('input.meta_data').val()
      };
      Recurly.post('/success/url', data, {});
    }

    // Do not specify successURL
 });

// ruby backend:  verify_subscription! params[:subscription]