Having an application handle a message

[Veregon]

Hi, I have an application which brings up a property sheet before creating any documents if it is being run for the first time. I'd like to have the App handle when the user presses Apply/OK in the property sheet. I've overridden the OnApply() in one of the property pages and want to have it send/post a message that the app can process to do the data exchange needed. How can I send the message so the app will see and process it?
Thanks