If SF (Sound Forge) is still on an old PC, your SF serial number is displayed in the menu "Help> About Sound Forge Pro". To get your serial number from the on-line Magix Service Center, you must log-in using the email address that was used when SF was initially registered. If you have a purchase receipt, or other purchase info, contact the Magix Customer Service Sales Team.

btw, welcome to the Magix Sound Forge forum @John-Scruggs.