Indie game storeFree gamesFun gamesHorror games
Game developmentAssetsComics
SalesBundles
Jobs
Tags

Find your download page (either through itch or through the original email with the purchase link) and there should be a button there that says "Get Steam Key" and that should do it!

https://itch.io/t/67604/how-to-get-a-steam-key-from-your-itchio-purchase if you need more info.