Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Reference
GV_HAIR_POINT_OBJECTParameter: Mode
GV_HAIR_POINT_MODEParameter: On
GV_HAIR_POINT_INPUT_ONParameter: Guide
GV_HAIR_POINT_INPUT_GUIDEParameter: Index
GV_HAIR_POINT_INPUT_POINTParameter: Segment
GV_HAIR_POINT_INPUT_SEGMENTParameter: Segment Delta
GV_HAIR_POINT_INPUT_TParameter: Position
GV_HAIR_POINT_INPUT_SET_POSParameter: Dynamic Position
GV_HAIR_POINT_INPUT_SET_DYN_POSParameter: Velocity
GV_HAIR_POINT_INPUT_SET_VELOCITYParameter: Mass
GV_HAIR_POINT_INPUT_SET_MASSParameter: Add Force
GV_HAIR_POINT_INPUT_ADD_FORCEParameter: Position
GV_HAIR_POINT_POINT_VECParameter: Tangent
GV_HAIR_POINT_POINT_TANGENTParameter: Velocity
GV_HAIR_POINT_POINT_VELOCITYParameter: Mass
GV_HAIR_POINT_POINT_MASSParameter: Force
GV_HAIR_POINT_POINT_FORCE