Possibilities could be...
1) During the "main" Inquisitor creation screen, you'd get to create your own Inquisitor, then *big surprise* another window (or another character slot option, or something along the line) shows up for you to create your Hawke and, maybe, your Origins Warden as well.
2) A unique-to-The Keep Hawke creation screen... but I would doubt that, what would happen then to people whom won't download The Keep? What if those players don't care about it or just never played past DA games before, and will only use Inquisition's default canon story? Yeah, food for thought...
3) During the actual playthrough, after the intro, after the regular Inquisitor creation screen. At some point in the game, perhaps from the war table or somewhere else in your Skyhold you'll learn that Hawke has been spotted somewhere, you'll have to investigate and before launching THAT specific mission you'll get a Hawke tarot card for customization, or something similar to that.
That's just what I think, but I'm surely wrong anyway, not sure how (other than the above).