Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Preview
ID_PAINTSPLINE_PREVIEWParameter: Position
ID_PAINTSPLINE_POSITIONParameter: Scale
ID_PAINTSPLINE_SCALEParameter: Rotation
ID_PAINTSPLINE_ROTATION