I have just been chatting with customer support and they asked me to post my issue here, too:
Today I got a message on Xbox Live with a code for an additional car pack for Forza 5. I bought the game some weeks ago directly after lauch of Xbox One via the online store. The package might be the day-one package...
Unfortunately, while reading the message, it got deleted before I could redeem the code.
Still hoping, the message can be regenerated...
At least the experience with chat-support was flawless!