save into ~/.oh-my-zsh/custom/plugins/tt/tt.plugin.zsh

tt.plugin.zsh

# lovingly ripped off the _tmux completion module included in zsh

function __tmux-sessions() {
    local expl
    local -a sessions
    sessions=( ${${(f)"$(command tmux 2> /dev/null list-sessions)"}/:[ $'\t']##/:} )
    _describe -t sessions 'sessions' sessions "$@"
}

function _tmux-attach-session() {
    [[ -n ${tmux_describe} ]] && print "Attach or switch to a session" && return
    local -a args

    args=(
        ':target session:__tmux-sessions'
    )
    _arguments ${args}
}

compdef _tmux-attach-session tt