Using hidden field to confirm status before postback

Declare in page
<input type="hidden" id="confirmed" name="confirmed" value="">

Set up javascript validation
function SelectBtn_Click(src, e)
{
  var f = document.MyForm;
  var v = f.NumberOfTxt.value;

    if (IsStringOfDigits(v) && confirm("Are you sure you want to   select " + v + "?\n If \"Yes\" click OK, otherwise click Cancel."))
  {
    document.MyForm.confirmed.value = "Y";
    event.returnValue = true;
    return true;
  }
  else
  {
    document.MyForm.confirmed.value = "";
    event.returnValue = false;
    return false;
  }
}

Set attribute in code behind
this.SelectBtn.Attributes["onclick"] = "return SelectBtn_Click(this, event);";

In postback, check the hidden value
"Y".Equals(Request.Form["confirmed"])