From Get snap.jpeg from UniFi Protect Cameras:

Get snap.jpeg from UniFi Protect Cameras
December 9, 2019

How to get the correct username & password for a UniFi camera that has already been adopted by UniFi Protect:

    Get into Protect UI https://protect.ui.com/
    Select NVR you’d like to use
    On left bottom corner click the Settings gear
    Click Advanced
    On the right side, you’ll see Device password, click REVEAL
    Copy the password
    Connect to the Camera IP through https, e.g. https://your.camera.ip.address
    Login with username ubnt and password from above
    Skip device setup, keep it in Unifi Video mode
    Enable the “Anonymous Snapshot” on the camera
    From there you should be able to access http://your.camera.ip.address/snap.jpeg