It seems that MS will add this function - window.startupTask - not only for converted desktop applications, but also for UWP applications.
You can see it at about 37:00 Tip, tricks and secrets: creating a great PC application for PC
But this feature is not ready yet - it will be available with the Windows 10 Fall Creators Update (I tried with SDK 16225, but not ready yet)
Added on 12/18/2017 . You can do this with the Autodesk Autodesk Win10 update. After publication, you will see the details.
Set up your login application (Windows Blog)
One point of consideration: for this function, you can simply "launch" the application - the application window does not appear. To see the application window, the user must click the application task in the taskbar. This is a bit ridiculous implementation for me. From the client’s point of view, “click to start” and “click to activate” are the same behavior. You can perform some task in the background before the user activates the application, but this is another story.
source share