Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Radius
MDATA_SPLINESKETCH_RADIUS
Parameter: Stroke Smoothing
MDATA_SPLINESKETCH_SMOOTHING
Parameter: Blend
MDATA_SPLINESKETCH_BLEND
Parameter: Create New Spline
MDATA_SPLINESKETCH_NEWSPLINE