You don't adopt it. It's a gift shop exclusive, meaning you get one only if you recharge real life money for ingame gems. It's stated on the description page of the promotion: you'll get one Premiere Neige for every 10 euros spent on the game.
Hello, to adopt Première Neige one has to make a purchase of at least 10€ in game (you can see the offers by clicking your jewels, to the left of your screen) these creatures are real money purchase Limited. 10€ makes you adopt one, 20€ gives you two, etc. Hope it help ^^ Good event to you !