Windows build on a mac

What's the problem? All you should have to do is select Windows as your target platform in the build settings. If it doesn't show up, then you didn't install Windows build support when you installed Unity and will need to run the installer again. You can find the installers for various versions here.

If you want to test out your windows build, you can export your game by selecting target platform to windows and Unity will automatically create a windows executable file for you.

You can test your build on your mac through these methods: 1) Installing third party windows emulation software for mac/

2) Using BootCamp service by Apple to create a separate windows partition to test your game with no performance decrease.