Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Soft Selection
HAIR_SOFTSELECTION
Parameter: Preview
HAIR_SOFTSELECTION_PREVIEW
Parameter: Tweak
HAIR_SOFTSELECTION_TWEAK
Parameter: Visible
HAIR_SOFTSELECTION_VISIBLE
Parameter: Type
HAIR_SOFTSELECTION_TYPE
Parameter: Coords
HAIR_SOFTSELECTION_SPACE
Parameter: Radius
HAIR_SOFTSELECTION_RADIUS
Parameter: Strength
HAIR_SOFTSELECTION_STRENGTH
Parameter: Clip
HAIR_SOFTSELECTION_CLIP
Parameter: Falloff
HAIR_SOFTSELECTION_FALLOFF
Parameter: Width
HAIR_SOFTSELECTION_WIDTH
Parameter: Falloff Curve
HAIR_SOFTSELECTION_FALLOFF_CURVE