I have an example
ffmpeg -i zs_712922_CT.mp4 -itsoffset 10 -i ct.m4a -map 0:0 -map 1:0 -c:v copy -preset ultrafast -y out1.mp4 (I don't know why this dropped the video, leaving only the audio.)
to
ffmpeg -i input.mp4 -itsoffset 10 -i ct.m4a -map 0:0 -map 1:0 -itsoffset 50 -i ct.m4a -map 0:0 -map 1:0 -c:v copy -shortest -y out.mp4