This is how I found out I had the Steam version, too... bought a copy of HW and my Mog Station was like "nope"...
luckily I hadn't bought it from the SE store, and the vendor I got it from gave me a refund so I was able to go and get the Steam version. All I can really suggest is that if Steam is unavailable is to get a friend to buy it on Steam for you (make sure the region matches with your ARR copy, of course) and then just send you the codes.