The portal recently got a coupon feature – you might even have some yourself. Pic from Ainheg:
Check if you have any coupons here (only on the EU server, though):
Official response from WG regarding the coupons (source: Madlollipop):
Shame there won’t be more coupon waves in the near future. Most likely users who haven’t played WoT in a long while were the ones who recieved these.