To watermark video using FFmpeg, run

ffmpeg -i SOURCE.mp4 -vf "drawtext=text='WATERMARK_TEXT':x=10:y=H-th-10:fontsize=32:fontcolor=white:shadowcolor=black:shadowx=2:shadowy=2" OUT.mp4

In the above command replace

SOURCE.mp4 = file name of the video you need to watermark.
OUT.mp4 = file name for the watermarked video.
WATERMARK_TEXT =  Text you need to watermark on the video.