If the software is still installed and running, you may be able to retrieve the key using a tool:

You can legally download for many Serif legacy products from archive.org (e.g., PagePlus X9 installer). However, you still need your own key to unlock it. No one can legally provide a key except Serif.

Here is the harsh reality most users face. Historically, Serif products asked for the key during installation, and then "phoned home" to a Serif activation server to verify the key wasn't blacklisted or overused.