If you actually own a retail box of version 6.0, the serial is typically found on the or the inside cover of the manual . Because it is so old, it may not even require online activation, but it often struggles to run on Windows 10 or 11 without "Compatibility Mode" enabled.
: For original physical copies, the serial number is typically located on the back of the Reviewer's Guide , the CD sleeve, or the printed manual. Digital Records Sound Forge 6.0 Serial Number