Windows external hard drive not detected in Startup Disk List
This procedure will not work for your 2015 Mac. Step 6.2.5 states: "Run: clean". The next step (6.2.6) states: "Run: create partition primary". This indicates the external drive is using the Master Boot Record (MBR) partition scheme. The 2015 and later Macs can no longer boot operating systems using this scheme. The question you refer to involved a 2014 Mac.

This procedure you reference requires you to download the "Windows 7 Automated Installation Kit" in step 3. Later, step 6.2.4 uses the imagex command from this kit. This command has been deprecated and should not be used with Windows 10. The replacement command is dism.exe, which is included in the Windows 10 installation iso. So steps 3 and 5.3 are unnecessary.

This procedure may be adapted for your model Mac. One change would be that the external drive needs to be using the GUID Partition Table (GPT) scheme.  This is probably not the only change needed. The accepted answer to the question "Unable to boot Windows 10 from Bootcamp external HDD" documents another possible change that may be needed.