According to the docs, it's not supported in the Desktop Class. You can only launch the browser itself.
https://docs.oracle.com/javase/7/docs/api/java/awt/Desktop.html
Why not use a ‘ShowPage’ action in your microflow? You could add the URL Redirector widget to the page.
If that won't work for you, I think you will have to approach this from the front, perhaps you can find a way to let a nanoflow handle it? The ‘openURL’ javascript action will open in the same tab.