How do I do that?Why not at system start from /etc/rc.local?
What do I put in this /etc/rc.local?Do you want something already running inside tmux? What's the objective?
/etc/rc.local you can create and set to execute and it is a shell script that will run at system startup.
Rc.local is executed after the scripts above, but before the rest of
the rc file is completed. In a default installation rc.local does not
exist, but its contents will be executed if the file is created by the
administrator.
tmux
and hit <enter> lxterminal --command="tmux"