If you have got the Windows platform yet, if you can get to Square-Enix digital store, get the Windows Digital CE and you should be given a registration key to register Windows platform.

Otherwise, find a local retail store, and lastly, you can try Steam - which will provide you a registration key, that maybe-could-be used in Mogstations "Add Windows platform" - however, be aware though, there's a huge confusion with Steam version and purchasing and using existing Service Account etc... Personally, I see less and less interest with Steam version now that I'm already up and going with what I have, I won't miss not having Steam Overlay TBH.