Mix seasonings and salt in small bowl; set aside. Pat turkey dry with clean paper towels. Rub 1 tablespoon of the seasoning mixture under skin of turkey.
Melt 2 tablespoons of the butter in Instant Pot on SAUTÉ setting. Place turkey, skin side down, in pot. Cook 8 to 10 minutes or until skin is browned all over, turning occasionally. Remove turkey from pot. Add potatoes to pot. Pour in stock.
Place turkey, skin side up, on top of potatoes. Sprinkle with remaining seasoning mixture. Close lid. Set VALVE to SEAL.
Set to cook for 30 minutes on MEAT/STEW function or until turkey is cooked through (internal temperature reaches 165°F). Vent pot to quick release pressure. Open lid once pressure inside has completely released. (Check manufacturer’s manual for safe operating directions.)
Transfer turkey to platter or carving board. Let stand about 10 minutes. Meanwhile, transfer potatoes to a serving bowl using a slotted spoon. Reserve turkey drippings to make Perfect Turkey Gravy, if desired.
Add remaining 2 tablespoons butter to potatoes and gently smash; stir to mix. Slice turkey and serve with smashed potatoes and Perfect Turkey Gravy, if desired.
Perfect Turkey Gravy: Remove turkey drippings from pot; measure 1 cup. Mix 2 packages McCormick® Turkey Gravy Mix and 1/4 cup flour in pot. Gradually stir in 3 cups water and 1 cup turkey drippings with wire whisk until smooth. Stirring frequently, cook on sauté setting until gravy comes to boil. Reduce heat to warm setting; simmer 5 minutes or until thickened, stirring occasionally. (Gravy will continue to thicken upon standing.)