use "drawtext" in the ffmpeg
for example :
ffmpeg -i input.mp4 -vf drawtext="fontfile=/path/to/font.ttf: \
text='Stack Overflow': fontcolor=white: fontsize=24: box=1: boxcolor=black@0.5: \
boxborderw=5: x=(w-text_w)/2: y=(h-text_h)/2" -codec:a copy output.mp4
