Preheat a 6-quart air fryer to 350°F. Make a sturdy 7-by-5-inch foil cradle with low edges and oil it lightly; keep it smaller than the basket so air can circulate.
Soak panko in buttermilk for 5 minutes. Stir in egg, onion, horseradish, salt and pepper, then fold in beef.
Shape a 6-by-4-inch loaf about 2 inches high on the foil and place in the basket. Cook at 350°F for 25 minutes.
Mix chili sauce and vinegar and brush over the loaf. Cook for 5 to 10 minutes more until the center reaches 160°F.
Lift the cradle carefully onto a heatproof plate and rest the loaf for 10 minutes.
Transfer away from collected juices and cut into 4 slices.