Passing values between user controls is a general ASP.NET task, which is the developer's responsibility, and is not directly related to the Telerik Controls.
Yet what I would suggest is that when you open the RadWindow you pass a reference to the desired element which you obtain from within your user control so that you can use it freely in the RadWindow. For that purpose you can use the RadWindow's argument as described in the following article: http://demos.telerik.com/aspnet-ajax/window/examples/dialogreturnvalue/defaultcs.aspx.
For your convenience I have created and attached a simple page illustrating the approach. Some points of interest:
- I added the client click event handler to the button in the code-behind since it also requires server-side coding to get the unique id
- For the sake of simplicity I have hardcoded the value, but you can use your own existing logic here easily
All the best,
the Telerik team