The Radio Tower event is a pre-determined event in the game that happens automatically:
1. After defeating the Elite Four and becoming Champion: The Radio Tower will become active and you will hear strange noises coming from it.
2. Visiting the Radio Tower: Upon entering the Radio Tower, you will encounter Team Rocket members and eventually face off against Giovanni.
3. Defeating Giovanni: After defeating Giovanni, the Radio Tower event ends and you will gain access to the tower's top floor.
There are no specific actions you need to take to activate the event; it happens naturally as part of the game's storyline.