Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Type
MDATA_SPLINEPEN_MODEParameter: Edit Tangents Mode
MDATA_SPLINEPEN_TANGENT_MODEParameter: Lock Tangent Rotation
MDATA_SPLINEPEN_LOCK_ROTATIONParameter: Lock Tangent Length
MDATA_SPLINEPEN_LOCK_SCALEParameter: Create New Splines
MDATA_SPLINEPEN_CREATE_SPLINES