Need help with form personalization on Sales Order Summary Form

I want to raise error during 'Book Order' action when a particular field is null on Sales Order Lines. I got the event name from Help → Diagnostics → Custom Code → Show Custom Event.

I have made the following changes which works when I click on Open in Order Organizer and try to Book Order. However, it doesn't throw error when I perform Actions → "Book Order" from Order Organizer Form.

Any help would be much appreciated.