There occurred some compatibility issues under Windows with the GPU.
Windows neither found the GT 710 nor a GTX 970 which I plugged in for testing purposes so I got a blackscreen all the time.
Furthermore the video signal from the boot process is not pictured bc of no bios support.
How to get the display output via GPU working on Windows?
1. Boot without dedicated GPU.
2. Installing windows.
3. Downloading NVIDIA DRIVER
4. Extract driver and look up the nv_dispwi.inf
5. installed it manually with pnputil
6. Reboot with grafics card.
7. now it’s working in Windows