You can find more information about the process of redeeming codes in this article. Please confirm with your retailer when you will receive the code.
Once the code is redeemed, you will receive an email to the mail address linked to your Ubisoft account. This mail will provide you with a key that will allow you to get access to your content.
Once the game is released and if you have successfully redeemed the key, you should get access to the Imperial Pack.