Hi Somer,

You can use OnClick in ClientSideEvent for confirmation and use CommandClick for server-side event. Here I put an attachment of my simple sample, please find a file named ConfirmationBeforePostback.rar. I hope this sample has appropriate with your need.

Regards,

Julia

Dear Julia,
Unfortunately, this does not work; as both clientside script and postback executes in parallel.
If you check from attached modified sample; you will see that whether you confirm or not, postback event runs.

Hi Somer,

I'm sorry but it seems that I need more time to investigate your scenario. To meet requirement of your scenario I should check the availability by using WebDialogBox, since the alert I used just work as confirmation and couldn't be used as optional confirmation.

Regards,

Julia 

Attached is the solution proposed by Julia, using WebDialogBox as confirmation and calling click event handler for the confirm button in the WebDialogBox.