This is a personal statement from me and not an official announcement or so. I just got back from the venue.
Please check carefully. The skin code is printed on a small card that is loose in the bag next to the box. When you pull the box out of the bag it might fly out.
Mine was stuck to to the box.
If you still can not find it I suggest that you contact player support:
Use a subject like championship skin code missing. It would most likely help if you provide proof up front that you attended the event. Please give us some time to investigate.
Thank you for posting! I'll check again (probably 15th time) but I don't think mine ever contained the code. already submitted a ticket hopefully it works out.