Hi Ramsay, thanks for replying!
They guide seems to be out of date: Steam doesn't let me access the CD key for FC3 anymore. If I right click on "DCS World Steam Edition" in my library I don't get any CD key option. And if I follow the "How do I find my CD key" guide (here: https://www.digitalcombatsimulator.com/en/support/faq/500/#1491005 ) it wants me to use a list of links at the right hand side of the Steam client - but those links are no longer there.
I think Steam has changed how their client works, and it's made it so that I can't find the CD key anymore! :(