Spray a large slow cooker with cooking spray. Add tomato sauce, ketchup, brown sugar, cider vinegar, Sriracha, garlic powder, and pepper flakes; stir until well combined.
Clean the chicken and tie the legs. Add to the slow cooker and turn several times to coat in the sauce.
Cover and cook on Low for 5 to 7 hours or on High for 3 to 4 hours. If possible, use a ladle to pour sauce over chicken every now and again to keep it moist. An instant-read thermometer inserted into the thickest part of the thigh, near the bone, should read 165 degrees F (74 degrees C).
Remove chicken from the pot. Shred meat into a bowl and remove/discard all bones. Put chicken back into the slow cooker and stir to combine with the sauce.