http://man7.org/linux/man-pages/man1/tmux.1.html#NAMES_AND_T...

    case $TERM in
    xterm*)
        echo -e '\033]0;INSERT TITLE HERE\007';;
    screen*)
        echo -e '\033kINSERT TITLE HERE\033\\';;
    esac

Based on what I do, but not in front of a computer to test exactly what I typed above.