Commands can't "stay" after rebooting the computer. When you shut it down and the power gets cut off, or when you reboot and the memory is cleared, things are gone from the memory.
You can, though, either keep the system state, by hibernating and resuming, or, what I expect to be the best solution for you, run the command after each boot.
As your command is probably something for X Windows, it must be executed after starting X Windows. Here I do that by adding the command to
~/.xinitrc, but that's for
startx, I think Ubuntu by default uses a graphical login screen, so you would need to use some other
~/.xsomething file. Perhaps