A Windows Service is a non-interactive process which runs in the background. There could be multiple users logged in to the computer, or there might not be any users logged in to the computer. Therefore, a service cannot show any user interface at all.
There used to be an option to allow services to interact with the desktop. However, this was a security issue, and has been disabled in all recent versions of Windows.
Your only option is to have an application running on the user's desktop which subscribes to an event on the WCF service and displays the appropriate UI:
How to: Create a Duplex Contract - WCF | Microsoft Docs