I found the attached image in a GOG thread.
I am trying to activate my Paradox game to access the Paradox forums properly.
I tried to do the steps shown in the picture to get my CD key, but it does not appear in the "More" dropdown menu on the game's page.
I am using GOG.com, not GOG Galaxy.
Please help.

If you have looked through all the options and text on the library menu - remember it could be under other or one of the other options - then contact support.

Not all Paradox games here have a CD key available. It's the same on Steam IMHO - some games just don't have CD keys, so unfortunately you cannot add them on Paradox.