It’s not so much where as when.
Our Stick-Up Cam doesn’t yet show this feature either, even though our Ring Doorbell Pro does, so it’s likely a matter of time before an update to the firmware is downloaded into the Stick-up Cam and an update to the Ring iPhone/Android app becomes available. When it does become available it will likely be in the Motion Detection Setup section of the Ring app.
Perhaps someone from Ring reading this can respond with an estimate of the timing as to when the “People Only” feature will be implemented in the Stick Up Cam.