This is for having a Windows Installation on a USB that you can plug into any computer.

sudo apt-get install virtualbox virtualbox-qt

Download virtualbox extension pack from oracle website for the respective virtualbox version: https://www.virtualbox.org/wiki/Download_Old_Builds_6_1

Start virtualbox, install the extension through tools → extensions.

Create new VM, Windows 10, VDI disk image, set usb to USB3
install Home Edition from ISO file.

Once installed, download WinToUSB https://www.easyuefi.com/wintousb/
plug in usb device, unmount, attach it to virtualbox

NOTE: doesn't work! gets stuck in the bootloader but never boots into windows from the usb stick!