It sounds like you already have this problem solved, but just in case:
Back when I still used Windows, the workaround was just to make sure you didn’t have a network connection when installing the OS. This could be done by unplugging the NIC during install on a physical system or by deleting the NIC in a VM (or other ways, obviously, but these were the simplest). It still took some convincing, but then you could make your username whatever you wanted.
I don’t know if that still works - it’s been years - but if you still use Windows and still have that problem, this might be worth a try.