Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Reference
GV_HAIR_PROXIMITY_OBJECTParameter: Mode
GV_HAIR_PROXIMITY_MODEParameter: Point
GV_HAIR_PROXIMITY_INPUT_POINTParameter: Index
GV_HAIR_PROXIMITY_POINT_INTParameter: Guide
GV_HAIR_PROXIMITY_POINT_GUIDEParameter: Segment
GV_HAIR_PROXIMITY_POINT_SEGMENTParameter: Position
GV_HAIR_PROXIMITY_POINT_VECParameter: Segment Delta
GV_HAIR_PROXIMITY_POINT_T