Node Custom Properties - Custom Node Events

Hello,

I have created a Node Custom Property. I need to trigger an alert that executes an external program when that Custom Property changes. How can I create a "Node Event" that my custom property has changed so that I may trigger my alert. For the built-in properties, Department, Contact, etc. there is an option to trigger based on a change. I need to do the same for a Custom Property.

Thanks for your help.