This is based on my own experience of installing Windows 10 on an external drive:
Step 1: Download a virtual machine to run Windows on. I used VMWare Fusion and used a 30-day trial. You do not need to do this if you already have a computer with Windows 10
Step 2: Search "windows 10 iso image" and click the first link from Microsoft. Download it.
Step 3: Open the VMWare application and upload the Windows 10 iso image onto it, and run Windows. You do not need to do this if you already have a computer with Windows 10
Step 4: Go on your virtual Windows and download WinToUSB from this: https://www.easyuefi.com/wintousb/.
Step 5: Open the WinToUSB tool and upload the same Windows 10 .iso image onto it, and choose which version to install. Connect your external drive to your Mac, make sure it connects to VMWare, and select it once it shows up on WinToUSB.
Step 6: Wait until the tool installs Windows 10 onto the external drive. While this is running, go to the Bootcamp app and ONLY download the Windows Support Software.
Step 7: Once the WinToUSB tool finishes installing, you can close out of VMWare by shutting down from the virtual Windows. The connected external drive should show up on your mac again, and drag the Windows Support Software folder into the drive.
You should now have an external drive with Windows 10 on it, and the Windows Support Software on it as well. You can boot from it by restarting and holding
option and choosing the external drive. Use the Windows Support Software to install the correct Apple Drivers in the external drive when inside the Windows 10 boot.
This was the method that worked for me on an Early 2014 MacBook Air on High Sierra and should work on yours unless something in Bootcamp Assistant changed.