How to install Windows ISO on a single partition of external HD on Mojave?

What it says on the tin. BalenaEtcher straight up doesn't offer the option to select individual partitions and UnetBootin... well, straight up will not open. I click, it'll ask if I'm sure I want to open it, and then the icon will pop in and right out of the dock.

Also, what I'm doing this for is to attempt installing Windows 7 on a partition of the boot drive and then upgrade it to 10 from inside the install itself so I can 1. Work with a sub-4GB ISO file which apparently makes software happier and 2. Upgrade from pirated to legit Windows. There very possibly may be flaws with this plan (especially since the Boot Camp drivers in another partition of the drive in question are made for Windows 10) so if there are feel free to let me know.

Solution 1:

Your model Mac is suppose to be Windows 7 compatible. Windows 7 should include SP1 or later. The Boot Camp Assistant included with macOS can be used to install Windows. There is a chance Mojave will not allow the installation of Windows 7. In this case, the easiest solution would be to install High Sierra to a new volume in the same APFS container as Mojave. You can then use the Boot Camp Assistant from High Sierra to install Widows 7. After Windows 7 SP1 is installed, you can remove the High Sierra volume. The Boot Camp Assistant will probably inform you that you will have to manually copy the Windows Support Software to the flash drive. There will be a link to the website with further instructions. After installing Windows 7, you can upgrade to Windows 10. After you successfully active Windows 10, you can elect to use the Boot Camp Assistant to remove Windows and reinstall a clean version of Windows 10. This version should automatically activate. Also, the clean install of Windows 10 should UEFI boot.