I THINK what will happen in that they will send you the code via email after the game has been dispatched. I read that somewhere either on here or on an official post in the Total War forum. Don't worry my man, you will get the code!
In my recent experience, you may not even have to wait for "the" email. When you enter the retail Steam code into your library it may automatically encorporate any and all DLCs associated. You don't (usually) have to enter pre-order DLC codes seperately, as that retail code will indicate it is of "batch # pre-order." I've only had one exception to this so far this year (DI Riptide) with pre-orders...
Just been on the phone to customer service and they are sending out all DLC codes via e-mail, we should see them by the end of the day. Failing that he said they were being sent within 48 hours of the dispatch
Really annoyed Amazon hasn't sent the code or even explained about the delay for this code - wouldn't have bothered to preorder without this bonus (still would have got game) They'd better do something today!