Cut 1 pound boneless, skinless chicken thighs into 1-inch pieces, then toss with 1 teaspoon garlic powder, 1 teaspoon onion powder, and 0.5 teaspoon black pepper until evenly coated.
Place 2 cups rinsed long-grain white rice in the rice cooker pot and arrange the seasoned chicken pieces on top of the rice.
Pour in 2.5 cups chicken broth and 2 tablespoons soy sauce, then gently stir to submerge the rice without disturbing the chicken layer.
Sprinkle 1 cup frozen mixed vegetables over the top without stirring to allow them to steam.
Close the lid and select the "white rice" or "cook" setting; cook for about 25–30 minutes, depending on your rice cooker.
When the cooker switches to "warm," let the meal rest, lid closed, for 5–10 minutes to redistribute steam and achieve fluffy rice.
Open the lid carefully, fluff everything with a fork, garnish if desired, and serve hot.