如何从JavaScript激活ASP.NET自定义验证器？

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:CustomValidator ID="CustomValidator1" runat="server" ControlToValidate="TextBox1" ErrorMessage="自定义验证失败" OnServerValidate="CustomValidator1_ServerValidate"></asp:CustomValidator>

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
{
    if (args.Value.Length < 5)
    {
        args.IsValid = false;
    }
    else
    {
        args.IsValid = true;
    }
}

    function ValidateCustomValidator1() {
        var validator = document.getElementById('<%= CustomValidator1.ClientID %>');
        if (validator) {
            ValidatorValidate(validator);
        }
    }
</script>

    function ValidateForm() {
        ValidateCustomValidator1();
        if (Page_IsValid) {
            // 提交表单
        }
    }
</script>