Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Mode
GV_HAIR_SELECTION_MODE
Parameter: On
GV_HAIR_SELECTION_OPERATOR_ON
Parameter: Iteration
GV_HAIR_SELECTION_OUTPUT
Parameter: Count
GV_HAIR_SELECTION_OUTPUT_COUNT