Yes I had a hard time finding it too but here it is. In App Store Connect select your app. On the top of the page next to your app name there is a Services link. Click on that. Then there is a Promo Code link on the left side of that page. Hope this helps you!
SteveV
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Thanks for this info, it also worked for me and the upload was much faster!
Awesome this worked for me! Thanks for your help on this.