I got an extra key from Humble, just like almost everybody else who was fast enough to claim it after the first LEGO Hobbit giveaway event finished early...
You have to activate the key before "January 3rd, 2019 @ 10:00 am Pacific" according to Humble, that's January 3rd, 2019 @ 19:00 CET and January 3rd, 2019 @ 18:00 UTC.