Hey! Thank you for buying the game!
in order to claim your steam key you need to:
1. Go to the game's itch.io download page
Alternatively, you can go to this page, and enter the email address you used when making the purchase: https://itch.io/docs/buying/already-bought
That should generate an email with a link to your purchase page.
2. On the download page, scroll past the downloads to the field that says "Claim your Steam key" and click the button "Get Steam key". The Steam key should be a 15-digit code.
Let me know if that works.