Yes, "Where No Man Has Gone Before" is the episode I meant.
"Charlie X" would be another useful example (I did say that my list was probably not comprehensive). Roddenberry did receive story credit for that episode based on a one sentence story idea entitled "The Day Charlie Became God." It read, in full, "The accidental occurrence of infinite power to do all things, in the hands of a very finite man." Fontana wrote the story outline (without credit) and the teleplay (with credit).