I preordered my copy of rise of iron a few weeks ago through Amazon to my surprise the code was released today, after a few clicks from my phone and entering my code my expansion was downloaded and now I can watch that sweet prelude which is very short and very familiar compared to the ttk one but hey I'm excited I wont have to stay up until midnight to download it because it's already been done, yay me!
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On PS4, and I'm pretty sure XB1 has a similar feature, you can set it to auto-downlaod when it becomes available. It still waits for the release date, so the Amazon method of getting the code early is probably quicker, but in case anyone is worried about having to log on and wait forever for the download, just set it to auto download and you should be okay.