Make the marinade by combining the ketchup, soy sauce, brown sugar, garlic, ginger, sriracha, and apple cider vinegar in a large bowl until the sugar dissolves completely. Save ¼ of the marinade for basting the chicken while grilling.
Add the chicken thighs to the bowl, mix until they are well coated with the marinade, cover with plastic wrap, and refrigerate for 30 mins to overnight.
Heat an indoor or outdoor grill to 325 F and grill the thighs for 5-6 minutes on each side, basting them with the reserved marinade. Let them rest for 3 minutes before serving. Serve with sliced pineapple and green onions.