From PGE Wiki
OnCustomCheat,0,0,ONCE?,EVENT,Active time,CHEAT STRING
When the player types CHEAT STRING, activate EVENT.
ONCE? = 0 = The cheat can be entered multiple times ONCE? = 1 = This command disables itself after first use.
// Activate event #1000 when typing "activate". Can only be done once OnCustomCheat,0,0,1,1000,0,activate