Need help with Deluge syntax for invoice item_custom_fields

Need help with Deluge syntax for invoice item_custom_fields

I need to set the value of an Invoice item_custom_fields 

The api_name is "cf_logo" and the value is from a custom picklist.

1. How can I get a list of all the "selected_option_id" values for the custom field "cf_logo"? 
2. What is the complete PUT syntax to add it to my line Items map

Here's my attempt which fails
  1. // Make another map for the invoice line items
  2. invoice_item = Map();
  3. // The ID stored in the Deal subform is the zcrm_product_id. Use this to find the item_id in books.
  4. itemid = element.get("Product").get("id");
  5. zcrm_product_id = zoho.books.getRecords("Items",books_organization_ID,"zcrm_product_id=" + itemid,"books_items_all").toMap();

  7. // Assemble the Invoice line item
  8. invoice_item.put("item_id",zcrm_product_id.Get("items").get(0).toMap().get("item_id"));
  9. invoice_item.put("quantity",item.get("Quantity").toString());
  11. custom_fields = list();
  12. custom_field_size = map();
  13. custom_field_size.put("Size", "cf_size");
  14. custom_field_size.put("value", ifnull("2274626000003483460",""));
  15. custom_fields.add(custom_field_size);
  16. invoice_item.put(custom_fields);