Thanks for the reply Princy.
The RadWindow I am trying to open is defined on the parent page and contains a contenttemplate. I need to specifically open the instance defined on the parent page and not a new RadWindow with a URL.
I tried this from server side but the window does not display unless I set VisibleOnPageLoad = true but then the page continuously displays on postback:
RadWindow radWin = (RadWindow)this.Parent.FindControl("winListings");