Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Start Radius
PRIM_HELIX_RADIUS1Parameter: Start Angle
PRIM_HELIX_STARTParameter: End Radius
PRIM_HELIX_RADIUS2Parameter: End Angle
PRIM_HELIX_ENDParameter: Radial Bias
PRIM_HELIX_RADIALBIASParameter: Height
PRIM_HELIX_HEIGHTParameter: Height Bias
PRIM_HELIX_HEIGHTBIASParameter: Subdivision
PRIM_HELIX_SUB