Use their "Lost Serial Number" tool by entering the email address you used to purchase the software.
If you're looking for free or open-source alternatives, consider: Globalscape Cuteftp 8 Serial Number Free
The most popular open-source FTP solution. It’s updated constantly, supports FTP, SFTP, and FTPS, and works on Windows, Mac, and Linux. Use their "Lost Serial Number" tool by entering
"Unlocking Globalscape Cuteftp 8: A Guide to Understanding Serial Numbers and Legitimate Alternatives" and works on Windows
: A high-quality Windows-based client that excels in SFTP and SCP transfers.