Well, it's not on sale, is it? Since you're not selling the game here. But look on your game's edit page, under App store links. (Edit: below the tags.) You can use those to show a nice big button that sends people to Steam, and that's that.
Of course, it would be even better if you sold the actual game here and offered a Steam key on the side. We have an external key system just for that. But if you don't want to, that's fine.