--------------aspx--------------------
function showMsg(ctl,args)
        {
            if(confirm(document.add.txt_title.value))
            {
                args.IsValid=true;
            }
            else
            {
                args.IsValid=false;
            }
        }
.................
<asp:textbox id="txt_title" runat="server" CssClass="stedit" Width="240"></asp:textbox>
<asp:textbox id="txt_hidden" runat="server" CssClass="stedit" Width="0" ReadOnly="True"></asp:textbox>
..................
<asp:CustomValidator id="Customvalidator2" runat="server" OnServerValidate="Val2" ClientValidationFunction="showMsg" ControlToValidate="txt_hidden" Display="Dynamic"></asp:CustomValidator>
--------------------cs----------------------
...................
public void Val2(object sender, ServerValidateEventArgs e)
        {
            e.IsValid=true;
        }
...................
private void Page_Load(object sender, System.EventArgs e)
        {
...................
            if(!Page.IsPostBack)
            {
                txt_hidden.Text="init hidden textbox";
...................
            }
        }