Mr. Halloway is asking for something that a real carnival character named Mr. Electrico “granted” to 12-year-old Ray Bradbury, which he never forgot. In Sam Weller's biography, The Bradbury Chronicles, the carnival's visit to Waukegan coincided with the funeral of Bradbury's uncle Lester Moberg, who had been fatally shot in a random holdup.
Mr. Electrico not only touched the boy with an electricity-charged sword and shouted "Live forever!" as part of the show, but the next day took him around the carnival, introduced him to the sideshow freaks, and told the boy that the two of them had known each other in a previous life. "You were my best friend in France in 1918, and you died in my arms in the battle of the Ardennes that year," the older man told the boy. You can imagine the deep impression this sort of thing would have made on a 12-year-old.
"Just weeks after Mr. Electrico said this ["Live forever!"] to me, I started writing every day. I never stopped."
See also the Bookmark notes on the carnival characters listed in chapter 7 (page 34 in this edition).