Hi, Ldee247.
When you purchase VoiceAttack through Steam, no registration key is necessary (it’s one of the perks of the Steam version over the standalone version from the VoiceAttack.com site). All you need to do is run the Steam client and launch VoiceAttack. Once VoiceAttack is launched once, you won’t even need to run the Steam client to run VoiceAttack (just run it from its installed location).
Hope that helps!