Oh, that's because you're running SSH under the hood. I see what you mean, having the port be passed to SSH would be better from a UX standpoint, but the thing is that mosh doesn't do everything SSH does, so it would need to fake every SSH option and then pass them through. As it is, you only need to learn --ssh and you can do "--ssh=ssh -D 8223 -L 1234:localhost:1234" etc.