Display alert message box when ASP.Net gridview button is clicked

protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        string message = "Type Your message on alert Over Here.";
        System.Text.StringBuilder sbuilder = new System.Text.StringBuilder();
        sbuilder.Append("<script type = 'text/javascript'>");
        sbuilder.Append("window.onload=function(){");
        sbuilder.Append("alert('");
        sbuilder.Append(message);
        sbuilder.Append("')};");
        sbuilder.Append("</script>");
    }