Hi,
So login to your NVR and obtain the RTSP URL by clicking on the camera and expanding the RTSP service
In the snapshot UTL put the following: api/2.0/snapshot/camera/XXXXXXXXXXXXXXXXXXXXXXXX?force=true&apiKey=YYYYYYYYYYYYYYYY
Where X = the part of the RTSP URL after the port number (after the forward slash) (I removed the _1 at the end of the URL)
and Y = API KEY which is obtained from clicking your user name in the top right of the NVR, settings, API Access