https://askubuntu.com/questions/296377/how-do-i-change-the-default-shell-used-in-byobu-tmux

Put the following in your $BYOBU_CONFIG_DIR/.tmux.conf (usually ~/.config/byobu/.tmux.conf or $HOME/.byobu/.tmux.conf):

set -g default-shell /usr/bin/zsh
set -g default-command /usr/bin/zsh