I saw someone created a community level where you can start with a boss unit. For reference

https://www.warzone.com/SinglePlayer/Level?ID=1188934
https://www.warzone.com/SinglePlayer/Level?ID=1184197

I was wondering how to do the same. I tried the custom scenario where you can add special units but it seems all I can add is a commander. Many thanks

but it seems all I can add is a commander

If that's what you saw, then it sounds like you weren't editing a community level. Go to Single Player -> My Levels -> Edit a level, then from there you'll be able to add bosses.

thanks Fizzer. Not sure why, but previously it was only a commander option. I now see the boss options