Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Type
MDATA_SPLINEPEN_MODE
Parameter: Edit Tangents Mode
MDATA_SPLINEPEN_TANGENT_MODE
Parameter: Lock Tangent Rotation
MDATA_SPLINEPEN_LOCK_ROTATION
Parameter: Lock Tangent Length
MDATA_SPLINEPEN_LOCK_SCALE
Parameter: Create New Splines
MDATA_SPLINEPEN_CREATE_SPLINES