Under tmux, change window in Irssi with meta key Alt + Num? I think this is specific to OS X Terminal.app

Viewing many windows in irssi via tmux.

I can use alt+right arrow to move windows as the metakey.

but alt+5 only produces [KH^
Found the answer moments after posting here.

Apparently, Terminal.app does not escape the alt key. It can be turned on in preferences by selectin Use Option as Meta key