Season the chicken. Combine the salt, garlic powder, onion powder, sumac, and coriander in a small bowl. Pat the chicken dry with paper towels. Season on all sides with the salt mixture — this can be done the day before, several hours in advance, or 30 minutes before cooking (refrigerate if salting for more than 30 minutes). Let sit at room temperature for 15 to 20 minutes while the grill heats. Meanwhile, make the sauce if using.