You will need ffmpeg installed.

Use the xrandr to discover your monitor, copy the resolution + offset, eg 1366x768+1920+113 and pass it to the ffmpeg command below.

Run the command pactl list short sources. Get a monitor, like alsa_output.pci-0000_00_1f.3.analog-stereo.monitor and pass it to the ffmpeg command below.

Run the command:

ffmpeg -video_size 1366x768 -framerate 60 -f x11grab -i :0.0+1920,113 -f pulse -ac 2 -i alsa_output.pci-0000_00_1f.3.analog-stereo.monitor output.mp4

Notice that there's resolution + the offset. Very important.