Preheat the electric smoker to 250°F. Add applewood chips according to the smoker’s instructions, and place a drip pan beneath the cooking rack if your smoker uses one.

In a large bowl, stir together the breadcrumbs and milk. Let them sit for a few minutes so the breadcrumbs absorb the milk.

Add the ground beef, egg, chopped onion, Worcestershire sauce, salt, black pepper, garlic powder, and 2 tablespoons of the ketchup. Mix gently with your hands until just combined. Avoid overmixing, which can make the meatloaf dense.

Shape the mixture into a compact loaf about 8 inches long. Place it on a smoker-safe rack or in a shallow smoker-safe pan, leaving room around it for the smoke to circulate.

Smoke the meatloaf until its center reaches 150°F, about 1 1/2 to 2 hours depending on its thickness and your smoker. Check the temperature with an instant-read thermometer.

While the meatloaf smokes, combine the remaining ketchup with the apple cider vinegar and brown sugar in a small bowl. Stir until the sugar dissolves.

Brush the glaze over the meatloaf when its center reaches 150°F. Continue smoking until the center reaches 160°F. Let it rest for 10 minutes before slicing so the juices settle.