How to add a caption with video using ffmpeg?

I have a video in.mkv which is dual audio and has built-in subtitles. I want to convert it to output.mp4 and I used the following command:

ffmpeg -rtbufsize 150M -i entrada.mkv' -acodec aac -map 0:0 -map 0:2 -vcodec h264 -c:s mov_text -map 0:3 "saída.mp4"

With this command I was able to add a caption, but it was not recorded in any video, so I had a good choice. Video is not a video being part of the video itself.

How I do? Image

I tried the command

ffmpeg.exe -rtbufsize 150M -i 'entrada.mkv' -acodec aac -map 0:0 -map 0:2 -vcodec h264 -vf:0:3 "saída.mp4"

But there is the mistake

At least one output file must be specified