Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Min Input
FIELDLAYER_RANGEMAP_MININ
Parameter: Max Input
FIELDLAYER_RANGEMAP_MAXIN
Parameter: Min Output
FIELDLAYER_RANGEMAP_MINOUT
Parameter: Max Output
FIELDLAYER_RANGEMAP_MAXOUT