You can't enter this as a promo code though. You have to enter it as an authorization code through the account management screen. Follow the directions they give you on the site.
Copied from their site:
With the recent release of version 7.1 all players get a special gift. The unique premium tank, lend-lease version of the Light Mk VII, Tetrarch including a free garage slot is ready to join your tank company in garage!
To redeem the tank you need to go to your Account Management page and enter code:
Please mind the following rules:
Use CAPS to enter the code;
The code may only be activated once (one account – one code activation);
The code is valid until 23:59 PST, February 3 (7:59 UTC, February 4).
More detailed information on how to activate bonus code you may find here.
Get your new tank and stay tuned to World of Tanks!