You need to participate in event which is run by SDE and aims to provide a chance for players to get EBB.You need to complete certain mission in order to reach the goal.
Eg, In the last EBB event . If you want to redeem Giulio Cesare(1940) , you need to play World at War 100times with 40wins.After you had completed this requirment , you may redeem your favourite EBB5 for one.
Or you may participate in the EBB event in the alternative way , use olives via the item store.
Eg, in the last EBB event , the other way you may get EBB is to spend at least 200olives in one transation , you may receive a coupon that you may have a chance to receive a EBB coupon or other items.
Hope this information can help you.