From Xojo Documentation
A notification named aName has been received. The process ID of the sender is passed in the aPid parameter.
Used by the Listen and Notify PostgreSQL feature.
Notifications are sent using the Notify method.
The aExtraparameter may contain payload information provided by another PostgreSQL system. Xojo does not have a way to provide a payload to a notification. as a workaround you can manually call NOTIFY yourself like this:
This code in the ReceivedNotification event handler displays the notification that was received: