Form validation with "checkbox accept terms condition" field in C# (server side check)

I need to show an asterix to the correspondent checkbox when the user try to send the form without check the checkbox... I found something like this but is not working, like the normal others <asp:RequiredFieldValidator fields:

<asp:CustomValidator  ID="CheckBoxValidator" runat="server" onservervalidate="CheckBoxRequired_ServerValidate" Display="Dynamic" CssClass="error-text" ErrorMessage='*' ValidationGroup="save"> * </asp:CustomValidator>

And in the code behind I have:

protected void CheckBoxRequired_ServerValidate(object sender, ServerValidateEventArgs e)
{
  e.IsValid = YourCheckBox.Checked;
  labelMessage.Text = "Your reservation has been processed."; 
}