Validate field with script or on action - without fc or process designer

Hello guys,

I want to validate field Affected CI which user is entering but I have a problem an automatic script that escalates Interaction to Incident and that overrides field validation that would normally work.

Do you know if I can put some kind of validation on entering/update action so it stops user to go further until he corrects or fills button.

Additional thing is that field is not mandatory so I can't control it with a mandatory condition.

Best regards,