if you're using rpg maker 2003, then it should be as simple as using the "enter hero name" and then use a conditional branch checking if the "hero" who's name you changed has a "correct" name. You'll find the condition on page 2 of conditional branches, under "hero" and then "name is".
Of course, the hero you'll use should be one that isn't actually in the game, but just used for the sake of a password.
If you're using rpg maker 2000, I'm not so sure, since I "think" the enter hero name was bugged there or something. Not entirely sure though, been a long time since I used it.