If there is a way that works on SP18/SP19 I would love to hear it.
Honestly, sometimes I don't know what on earth have the engineers of HP thinking. The excuse of why user notification message is blocked on field change is pretty lame.
The concept of notification message is pretty useless if you don't allow messages to be presented on field change.
Do you Read HP? Notification messages are more usefull well you notify IMMEDIATELY on user error than waiting to fill 50 fields and popup en error message after trying to save the item. This confuses users and all users are NOT technical people.
Have you tried this. Create the UI rule with the trigger "When a record is saved". Create the notification message and then save the UI rule. Then edit it and change the trigger to "When a value has changed" and then go into the conditions and turn on "Evaluate when this field changes". It has worked upto SP17 - does this not work on 18.