To launch a command in a new byobu window
byobu new-window "./mon_script.sh param1 param2"
To start a new session (new byobu instance you can attach to) with a command you can:
byobu new-session -s "session name" "ls -la && sleep 5"
To create it already detached:
byobu new-session -d -s "session name" "ls -la && sleep 5"
To attach to a session by name:
byobu attach -t "session name"