Cant't update custom field when custom field is external lookup in Zoho Books

Cant't update custom field when custom field is external lookup in Zoho Books

Hello

I use that :
  1. po = zoho.books.updateRecord("purchaseorders",XXXX,purchaseorder_id,updateCustomFieldseMap,"el_books_connection");
  2. c_f_Map2 = Map();
  3. c_f_Map2.put("label","EL ORDER ID");
  4. c_f_Map2.put("value",el_order_id);
  5. c_f_List.add(c_f_Map2);
  6. updateCustomFieldseMap = Map();
  7. updateCustomFieldseMap.put("custom_fields",c_f_List);
This is working because "EL ORDER ID" is an number field, when trying doing the same with a lookup fiels it's not working
I try also with customfield_id instead of label, not working

Thanks for any ideas