Hey guys, I just spent some time figuring this out.
Running from windows. The butler command line docs say to use an equals sign --userversion=USERVERSION.
I was unsure whether the version was actually getting transmitted -- it'd be nice to have the user version displayed in the app and on the edit game page. Running butler status user/game:channel returns invalid game. In the end I visited https://itch.io/api/1/x/wharf/latest?target=user/game&channel_name=channel and saw that the user version was successfully updated after all.