How do yoyu get the CD key for a downloaded version of Halo Custom Edition?

Question

How do yoyu get the CD key for a downloaded version of Halo Custom Edition?

Answer 1

It uses the CD key from the full retail version of the game.   You need to have bought the original game in order to use it.

Halo Custom Edition is  sort of a multiplayer add-on for the full retail version of Halo.  You don't need to have the original game installed in order to play it, but you do need to have bought a copy of it, because HCE uses the CD key from it.