If you don't have a bootable ISO to install Windows, you can make a VHD from it with New Hard Disk wizard...

installing Win 10 into Microsoft Virtual Machine - no tutorial?-image.png

... by selecting Copy the contents of the specific physical disk:

installing Win 10 into Microsoft Virtual Machine - no tutorial?-image.png

Add new VHD to newly created VM (of course in addition to disk you will install Windows to), change the boot order in VM settings to boot from hard disk instead of DVD and install Windows from this VHD.