如何指定在显示ModalPopupExtender时运行的javascript

<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="Button1" PopupControlID="Panel1" OnShowing="ModalPopupExtender1_Showing">

protected void ModalPopupExtender1_Showing(object sender, AjaxControlToolkit.ModalPopupExtenderEventArgs e)
{
    // 添加JavaScript代码
    ScriptManager.RegisterStartupScript(this, this.GetType(), "showModal", "showModal();", true);
}

function showModal() {
    // 在此处编写功能
}