I just noticed that I have missed the fact that you are using the Prometheus version of the control - I apologize for that. In RadWindow Prometheus, the Callback property does not exist in the client-side API and cannot be used.
I assume that you have seen an older version of the demo, because this part of the code was removed in the latest update of RadWindow Prometheus - you can check that on our online demos. You will need to use one of the other 2 approaches to send the information back to the parent page.
Once again, sorry for the omission.
the Telerik team