/window -a #channel
Some other switches of the /window command (c,n,x,r,R,h) could be of use to you as well - though meant for custom windows in the first place, a lot of the options of /window and $window() do also work for channels, queries, "status window" and other "non-custom" windows. You can even use identifiers like $wid, then manipulate /window @<wid> .