If you bought from Amazon, it will be on the "Your Digital Items" page:
https://www.amazon.com/gp/swvgdtt/your-account/manage-downloads.html
I just checked mine and while I have not purchased Turbotax from Amazon, I did have an activation key for another product that I had purchased. I can't believe it wasn't on the page where you downloaded in the first place, you probably just didn't see it.
If you do not see it there, contact Amazon, not this board.
