In the Original Game, mystic codes are codes put in at specific areas in the game. Depending on what you choose, you may receive experience, money, words of wisdom, or a new party member.
The Codes and How to Get to Them:[]
There are two areas that prompt you to put in a mystic code. There are two ways to get to the first one- at the Owl Tree, take the bottom right exit and continue going right. Starclan will then speak to you, and you will be prompted to put in the Mystic Code.
Another way to get to this same area is through the Training Hollow. When you're there, go to the only left exit to the next map. Work your way through this map to the only left exit, then face the flowers going down in the second row.
The code for this area is 7193024
The second area you can be prompted to put in a mystic code is in the moors, heading towards Windclan Territory. You should naturally encounter it during the quest to find Windclan. The code for this area is 8150673.
Both codes can be used, and you can use them at any point in the game. However, you can only use them once per playthrough, so make your choices wisely.