How to get alert message before redirect a page in asp.net using c#

String scriptStr = "<script language='javascript' type='text/javascript'>";

                            scriptStr += "alert('Hi');";

                            scriptStr += "window.location = '";

                            scriptStr += "google.com";

                            scriptStr += "'";

                            scriptStr += "</script>";

                            this.Page.ClientScript.RegisterStartupScript(this.GetType(), "AlertMessage", scriptStr);