You can do something like this:
As soon as the player enters the map, the Autorun trigger kicks in and this event activates. It generates a random number between 0 - 99 and stores it in Variable 001 (you can choose a different variable if you want). Then the Conditional Branch checks if Variable 001 is less than 5, which means a 5% chance (again, you can change this value). The text inside the Conditional Branch is whatever your event is, and after that you need "Erase Event" so that the event will not trigger again until the player leaves and reenters the map.