If your motherboard is no less than 10 years old then it probably support UEFI.
Lookup BIOS for "Boot Mode" setting switch the setting Legacy to UEFI.
Then, Ubuntu installation will show you the "use as efi partition" option.
You'll have to switch BIOS to Legacy to boot in windows and switch to UEFI to boot with the portable ubuntu drive.
Switching BIOS every time is bit inconvenient.
For one solution, Windows 10 can be booted UEFI and one can easily switch windows booting option from legacy to uefi.(no reinstallation required. google "win10 convert legacy to uefi")
For the other way around, ubuntu can be installed to boot in legacy mode. Installing ubuntu with legacy booting is not that complicated but you will still have to reinstall grub.
I'm happy with UEFI booting for windows. So, I have no clue how to reinstall grub for legacy.
Download https://t.co/Pm8hp1qXGt