I just found it - it is still under the Services tab, after you select your app. But for some reason sometimes the Services tab doesn't show up for me at least. I logged out and logged back in (mac safari), and then it showed up.
This is from the App Store Connect docs (as of today, this Jan 30, 2023 ;)):
Request promo codes
From My Apps, select your app.
Click the Services tab.
In the sidebar, click Promo Codes. The Promo Code page opens with Generate selected.
Under App Promo Codes, enter the number of version promo codes and under In-App Purchase Promo Codes, enter the number of in-app purchase promo codes you want to generate.
On the top right, click Generate Code.
Read and agree to the contract terms for Apple to distribute free versions of your app or in-app purchase, and click Generate Code.
Click OK. You can view or download the generated promo codes on the History tab.
mobob
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