I find a "way" to turn it on. You need to connect another external monitor, additionally to the Apple Studio Display (ASD), so then you to have connected both ASD and the other one simultaneously. And then from the Windows settings, choose this additional display to be your main, then ASD turns on mirroring the other display. However this is not the "smartest" approach. Tested on two diffrent HP Elitebook 845 G8.
Best regards,