Harry's name was put into the Goblet of Fire by Barty Crouch Jr., who was disguised as Mad-Eye Moody using Polyjuice Potion. He used a powerful Confundus Charm to trick the Goblet into thinking there were four schools instead of the usual three, allowing Harry's name to be entered under a fictitious fourth school. Since Harry was the only candidate from this "fourth school," the Goblet had no choice but to select him as a fourth champion for the Triwizard Tournament, despite him being underage and not having entered himself
. This was part of a larger plan orchestrated by Voldemort and Barty Crouch Jr. to manipulate Harry into participating in the tournament and ultimately lead him into a trap to restore Voldemort's physical body. Many at Hogwarts initially believed Harry entered his own name out of ego, causing tension, but it was later revealed that only dark magic could have confounded the Goblet, and Harry was innocent of putting his name in
