If you can get a small portable SSD (like the SanDisk Ultra 500). https://www.sandisk.com/home/ssd/extreme-500-ssd
, it's much better (& safer) than sharing an internal disk for dual-booting
So, on the smaller version (120 GB) . . . there is plenty of room for the SSD & bootloader. When plugged in, it will show in the uefi boot menu and will allow Windows to be the sole OS on the internal hdd.
If installing in UEFI mode, be sure to create a 512 MB EFI partition, else, if running in Legacy mode (BIOS/MBR), then let the Grub bootloader be installed to the external SSD.
Note: works best if your host PC has usb v3. Then it runs fast just like on the internall hdd/ssd.