Cut Loom screen recording at 39s, trim video around the edges to hide scrollbar and Loom stop button using FFmpeg
Cropping is carried out using a video filter. Also, a filtered video has to be re-encoded. So the command would be

ffmpeg -i input.mp4 -vf crop=1824:1026 -t 39 -c:a copy output.mp4


See the docs for the crop filter for all options.