Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Inner Radius
PRIM_STAR_IRADParameter: Outer Radius
PRIM_STAR_ORADParameter: Twist
PRIM_STAR_TWISTParameter: Points
PRIM_STAR_POINTS