Usually you will add it as a property. In header:
UPROPERTY(VisibleDefaultsOnly, BlueprintReadWrite)
USceneComponent* DefaultSceneRoot;
And create it in class constructor:
// Default root
DefaultSceneRoot = CreateDefaultSubobject<USceneComponent>(TEXT("DefaultSceneRoot"));
RootComponent = DefaultSceneRoot;
DefaultSceneRoot->Mobility = EComponentMobility::Static;
DefaultSceneRoot->bVisualizeComponent = true;