The option would be to use the Brightness control in Video Settings to decrease the brightness of your video footage until the background is dark.
You may need to film with a black cloth under the piano bench.
Yeah i already tried that out, but if i decrease the Brightness, my Piano (Key bar) also gets dark and the keys aren´t visible anymore so my hands.
Then you would need to film with a dark cloth under the piano bench, so that you only need to decrease the brightness a small amount.
Alright i´ll try that out, Thank you for you quick answer