I was able to sub through Steam. The sub window automatically pops up if you go through steam to start the game and don't have a sub. I didn't sub again after the month expired, it didn't renew which I thought was a good thing. I will gladly jump back in when thievery is added.
Yes, it is true that you can sub by pressing 'Play' on Steam, and then get the pop-up at the game's main menu if no sub is active. I was suggesting that an URL can be added to the game's Steam web page (the one people go to see the trailers, screenshots, reviews, news, etc) that links directly to account management. Which adds more pathways for people to conveniently reach it, thus potentially generating more subs.