Hey! The serial number is found in the "My Library" section of your itch.io account.
- Click the little arrow in the top right corner of your screen
- Click "My Library"
- Click on "My Purchases" on the left
- Click on GuyEngine V2
- Scroll to the bottom where it says "Claim download key"
- Click the button that says "Claim download key"
- Write it down! 😁