The base command for adding watermark looks like this
ffmpeg -i input.mp4 -i watermark.png -filter_complex "POSITION_HERE" out.mp4
You can replace POSITION_HERE with any position you need from the following
Top Right
overlay=main_w-overlay_w-10:10
Top Left
overlay=10:10
Bottom Right
overlay=main_w-overlay_w-10:main_h-overlay_h-10
Bottom Left
overlay=10:main_h-overlay_h-10
Center
overlay=x=(main_w-overlay_w)/2:y=(main_h-overlay_h)/2