Warning. Spoilers. If you'd like to figure the code out for yourself, do not read onward
Inside of the Crafts Room in the Community Center, the player can come across a glowing tablet in the middle of the floor. Upon first noticing the scroll, you will see a Spirit walk towards it and then disappear. The scroll, when translated, reads:
"Crafts Room: We the Junimo are happy to aid you. In return we ask for gifts of the valley. If you are one with forest magic then you will see the true nature of this scroll."