How to permanently add subtitles to MP4?

Dear Experts,

I have two files:

somevideo.mp4
somevideo.ass

I can play these together with VLC player and see the subtitles.

But I would like to create a new .mp4 file with
the subtitles permanently merged/hard coded into it.

What is the secret to do this?

The simpler the tool, the better.

Thanks a lot!