Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Enabled
ID_FIELDLAYER_ENABLED
Parameter: Value
ID_FIELDLAYER_ENABLE_VALUE
Parameter: Color
ID_FIELDLAYER_ENABLE_COLOR
Parameter: Direction
ID_FIELDLAYER_ENABLE_DIRECTION
Parameter: Rotation
ID_FIELDLAYER_ENABLE_ROTATION
Parameter: Layer Opacity
ID_FIELDLAYER_STRENGTH
Parameter: Layer Blending
ID_FIELDLAYER_BLENDINGMODE