Maybe work this but i didn't try i'm not sure
Browser.ExecJS("
prepareRewardVideoAd(adId/options, success, fail);
showRewardVideoAd();
// onAdLoaded
// onAdFailLoad
// onAdPresent
// onAdDismiss
// onAdLeaveApp
document.addEventListener('onAdDismiss', function(e){
// handle the event
});
")