The license key is generated by Gumroad and it looks like this:
7BDE50F1-FE6545E6-A8FD5AD4-A7A300B7. After you bought the book you can find your key in several places:
It is included in the confirmation email you got when you bought the book.
If you bought the book before 2022 Oct then license keys weren't enabled. Try the next method in that case.
In your Gumroad account, find it in the Library/Product's download page.
Check it here: What is my license key?
Contact me directly at firstname.lastname@example.org.