I'm looking for a way to prompt the windows 10 system to lock the display in order to provide more privacy by protecting it from unauthorized use. Is there any standard API for doing this in Windows Universal development? Or alternatively to simulate Keyboard for sending Win+L combination.
I understand you probably want to do this for good intentions, but such functionality is not available via any APIs in UWP. The reason for this is very likely the fact that giving an app the ability to lock the PC could easily give a malicious app the ability to lock the user out of the system by launching on startup and always locking the PC anytime it is activated. The same way apps are unable to shutdown or restart the PC.
Furthermore, the keyboard shortcut cannot be simulated for likely the same reasons, as the Win key on the keyboard is in general reserved for system shortcuts and it is really not easy to override it (you have to perform really low-level keyboard handling like in this example, which is again not something that will be allowed to pass Microsoft Store certification).
I would suggest assuming that the user has the auto OS lock timeout set up the way he or she likes it and that if you want to provide additional security in your app, you can add some kind of PIN-lock mechanism so that your app will lock itself automatically when not used for certain period of time.