The following page of the Final Fantasy 7 Remake guide provides a description of Discovery - Vagabond Johnny to discover in Chapter 9 The Town that Never Sleeps.
While visiting the city in Chapter 9, at one point you encounter Johnny running through the city. Run after him-you don't have to worry because he can't get away from you, so calmly join the man after a while.
When you get to Johnny, talk to him and then choose the options to agree with the man.
Reward: Johnny will appear in subsequent side missions and in later chapters. For watching all the cutscenes with Johnny, you will receive the Johnny Experience trophy.