Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Enable
MDATA_SOFT_ENABLEParameter: Surface
MDATA_SOFT_SURFACEParameter: Rubber
MDATA_SOFT_RUBBERParameter: Restrict
MDATA_SOFT_RESTRICTParameter: Preview
MDATA_SOFT_PREVIEWParameter: Falloff
MDATA_SOFT_FALLOFFParameter: Mode
MDATA_SOFT_MODEParameter: Radius
MDATA_SOFT_RADIUSParameter: Strength
MDATA_SOFT_STRENGTHParameter: Width
MDATA_SOFT_WIDTHParameter: Spline
MDATA_SOFT_SPLINE