Loading Bar Until Ad Show Up

Hi guys!

I want to show a progress bar until Reward Based Video is loading and then showing but LoadRewardBasedAd i a void type so it does not return. How can i do the operation? Thanks for your help.

 private void LoadRewardBasedAd()
 {
 
 #if UNITY_EDITOR
 string adUnitId = "unused";
 #elif UNITY_ANDROID
 string adUnitId = "ca-app-pub-3940256099942544/5224354917";
 #elif UNITY_IPHONE
 string adUnitId = "ca-app-pub-3940256099942544/5224354917";
 #else
 string adUnitId = "unecpected platfrom";s
 #endif
 
 rewardBasedVideoAd.LoadAd(new AdRequest.Builder().Build(), adUnitId);
 }
 
 private void ShowRewardBasedAd()
 {
 if(rewardBasedVideoAd.IsLoaded())
 {
 rewardBasedVideoAd.Show();
 }
 }

 IEnumerator LoadAsynchoronously()
 {
 AsyncOperation operation = LoadRewardBasedAd();
 
 while(!operation.isDone)
 {
 Debug.Log(operation.progress);
 
 yield return null;
 }
 }