Director George A. Romero later became a frequent collaborator with King, working together on Creepshow and The Dark Half, but their seminal shared credit is Knightriders, which sees King briefly appear as a hoagie-eating audience member at a motorcycle dueling event.
(Fun bonus: the woman he is sitting next to in the shot is his wife Tabitha King.)
As you’ll find reading this feature, the vast majority of cameos that Stephen King makes are in movies or television shows he is associated with via either source material or direct creative involvement, but it’s funny that his first ever doesn’t fall into either of those categories.