Hi Thomas,
Here is one way to call the JavaScript properly:
More information on the subject is available in this blog post:
http://blogs.telerik.com/blogs/posts/09-05-05/executing_javascript_function_from_server-side_code.aspx
I also noticed that you have set a very high value for the Height property - that is why I would suggest to set the KeepInScreenBounds property of the RadWindowManager to true. This will ensure that when the window is shown, its toolbar will be visible in the viewport.
Best wishes,
Georgi Tunev
the Telerik team