Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Select From Polygroup
ID_ACTIVATEPOLYGROUPSELECTIONParameter: Mode
ID_OPERATIONParameter: Assign Polygroup
ID_ADDPOLYGROUP