Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: X(t)
PRIM_FORMULA_XParameter: Y(t)
PRIM_FORMULA_YParameter: Z(t)
PRIM_FORMULA_ZParameter: Tmin
PRIM_FORMULA_TMINParameter: Tmax
PRIM_FORMULA_TMAXParameter: Samples
PRIM_FORMULA_SAMPLESParameter: Cubic Interpolation
PRIM_FORMULA_CUBIC