Hey all, please do yourself a favor and go this wargaming page:
Two days ago I joined up with two random people in game and we went on to have a beautiful game (yes, we were the high tier's I know) where we got all 15 kills between us. After the game one of the other people i platooned with messaged me and let me know about this challenge. I messaged wargaming to see if we could be considered for the event in spite of clicking the code after the fact and they said no.
I know that the page explicitly tells everyone to sign up but I am not able to check the website every day to join up for such things and so I had not heard of it yet. It seems strange to me that they could not allow anyone to be a part of such an event as I am sure that the vast majority of players are probably in the same situation. I would have really loved to win so this result is unfortunate.