The RadWindow control doesn't have RadWindowManagerProxy.
You can add a public property in the MasterPage, which returns a reference to the RadWindowManager and use it in the ContentPage. You can find a simple example, which illustrates the described approach, attached to the thread.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.