This looks really cool, but they should make it clearer that you only get one submission per day, and they're not going to check that your answer is valid before submitting.

I submitted this (wrong) answer

1. e3 Na6 2. Bxa6 Nf6 3. Bf1 Ne4 4. d4

but only realised afterwards that it has to be _exactly_ 4 moves, less than 4 moves is not good enough.