Been asked to use Selenium to test Windows desktop application UI--how is this possible?
You can't. Selenium can only automate web browsers, and even then, only certain browsers (Chrome, Firefox, Internet Explorer, Opera, and occasionally Safari). It interacts with them at a very deep level, very specific to the browser in question, in ways that don't apply to other Windows applications.
Selenium is meant for GUI tests of web applications and nothing else. But you can include Sikuli in webdriver code and can test the any of the applications either desktop or web applications, since it uses Image recognition.
else if you want to test the desktop application using Selenium thne u need to import Autoit and can test those as well