Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Soft Selection
HAIR_SOFTSELECTIONParameter: Preview
HAIR_SOFTSELECTION_PREVIEWParameter: Tweak
HAIR_SOFTSELECTION_TWEAKParameter: Visible
HAIR_SOFTSELECTION_VISIBLEParameter: Type
HAIR_SOFTSELECTION_TYPEParameter: Coords
HAIR_SOFTSELECTION_SPACEParameter: Radius
HAIR_SOFTSELECTION_RADIUSParameter: Strength
HAIR_SOFTSELECTION_STRENGTHParameter: Clip
HAIR_SOFTSELECTION_CLIPParameter: Falloff
HAIR_SOFTSELECTION_FALLOFFParameter: Width
HAIR_SOFTSELECTION_WIDTHParameter: Falloff Curve
HAIR_SOFTSELECTION_FALLOFF_CURVE