Calling WebService on form Closing event

I want to update a database record if the user closes the Silverlight application.
In this case it is Out Of Browser.
I created an event which does fire when the application is closing and the
Async Call is hit with the debugger but the call does not actually happen.
I don't care about doing anything in the completedevent sub but I do at least want to make the last call
to the service function.
I assume that the form closing is preventing the call from actually making it to the service?
I read about a Application_Exit event but I couldn't get it to work and suspect I would get the same results as above.
I've also read that I may be able to make the call synchronously but no luck as well.
Any advice on how and if I am able to do this.
Thanks.

Dim appWindow As Window = Application.Current.MainWindow
AddHandler appWindow.Closing, AddressOf appWindow_Closing

Public Sub appWindow_Closing(sender As Object, e As System.ComponentModel.ClosingEventArgs)
LogOffUser(True)
End Sub