Mannequin is now launching on Steam! You can view our store page here: https://store.steampowered.com/app/1280420/Mannequin/

You can however, support this project via itch.io and get both the itch.io version and a Steam key. See the procedures here: https://itch.io/t/67604/how-to-get-a-steam-key-from-your-itchio-purchase

And the steps for activating it on your Steam account here:  https://support.steampowered.com/kb_article.php?ref=5414-tfbn-1352