Dude.
I'm not going to log in to my steam account via itch.io. That is just way to sketchy. If you want to live up to your promise you should make the key available from my itch.io account page. Didn't expect much to be honest, but his is kind of dissapointing.
Note that the steam log in page states that "Note that thingonitsown.itch.io is not affiliated with Steam or Valve". Not saying that this is intentional, but I for one wont jepoardise my steam login to redeem an 18 dollar game.