Simply type: cmd . and press Enter in the Far Manager's command line. Note that this does not really quit the Far Manager but starts a new cmd session within the currents Far Manager directory. If you type exit in that cmd session you will come back to your left Far Manager session. But at least you can use this cmd session in "fullscreen", instead of the command prompt in Far Manager, even "maximized" with CTRL+o (not like Midnight Commander mc on Linux, there you have the full terminal space) is stuck on the last line of the current terminal.