You really should have a separate system partition, so you will probably need to use a live linux disc and gparted to create a small 500-800MB partition on the same disk as Windows 10. If your using the MBR you'll create it as a normal primary partition or if you're using GPT then it needs to be a fat32 partition with the GUID c12a7328-f81f-11d2-ba4b-00a0c93ec93b for EFI boot partition. Then you would assign it a drive letter temporarily and run
bcdboot C:\Windows /s H: /f EFI where H: is the letter assigned the system partition or
bcdboot C:\Windows /s H: /f BIOS if the disk is MBR