ffmpeg – Adding custom tags to AVI video file

I use software that accepts another AVI file, but not mine. I converted my file to have similar properties, and currently, there is not much difference between the files, I think the only thing I have left to change is adding some specific tags.

So I need to add some custom tags to my AVI video file.
I can’t use any software that is supported only in Windows because I have a Mac. I tried using FFmpeg in a few variations. This is one of them:

ffmpeg -i input.avi -metadata mytag="some value" -c copy output.avi

I also tried AVI MetaEdit (and it’s not an option there) and used mkv file then converted it again.

There must be a way to add this tag somehow, as I can see it in my tags in the other video file.
Any ideas?
Thanks!