Hello,
do you mean when you develop a new form control or when you simply add a new record to the CMS_Country table?
If you create a new form control, you can specify the rules for IsValid condition. Please see here:
Developing form controls.
The validation fires automatically and it must meet the condition in the IsValid method to pass the validation.
If you add a new record, it uses the same validation as for other values of Country selector.
Best regards,
Helena Grulichova