I'm not sure if Moodle forms support that. It's certainly new to me.
If I need something unusual, I will implement a validation() method in my forms class. See https://docs.moodle.org/dev/lib/formslib.php_Validation
I'm not sure if Moodle forms support that. It's certainly new to me.
If I need something unusual, I will implement a validation() method in my forms class. See https://docs.moodle.org/dev/lib/formslib.php_Validation