I had to use a custom validator inserted dynamically from cold behind. One disadvantage is that it can’t utilize the popup dialogue that the rest of the form designer validators use, because I couldn’t find a way to integrate it into the form designer specific validator system. So, it I just used display=dynamic and it shows up as red text next to the form field. This is a pretty basic use case that the form designer should be updated to handle.
See here for details: How to use a .NET CustomValidator control with server-side validation