If you've purchased the game, you should get a Steam key. Go to your download page, and go to the "Claim Steam key" section. See this image:



If you do not see the "Claim Steam key" section, then please send an email to support@itch.io, telling them that you cannot see your Steam key on the download page of Patrick's Parabox, and tell them your account username or email address. This has happened to a handful of people, and itch support has said that in these cases, please contact them and they'll fix it. (And they do fix it!)