How can I move a Virtualbox window to another screen by keyboard?

You can move the window, however you need to make sure that the keypress is not consumed by the guest operating system.

Use Alt-Tab to set the focus onto the virtual machine. This window will immediately grab the keyboard, and subsequent keypresses are sent to the guest operating system. That is why you cannot switch back from it using Alt-Tab again.

You need to release the keyboard from the guest operating system by pressing the Host Key Combination. It is configurable but per default the Right-Ctrl key.

After you pressed that, you can use the Windows + cursor keys to position the window, or Alt-Tab to focus another window on the host operating system.