Create

// Create a Map to store all the data
Map<String, Object> itemMap = new Map<String, Object>();
itemMap.put('Name', 'GenWatt Diesel 200kWk');
itemMap.put('Code', 'GC10402');

Map<String, Object> salesDetailsMap = new Map<String, Object>();
salesDetailsMap.put('UnitPrice', '210');
salesDetailsMap.put('TaxType', 'OUTPUT');
salesDetailsMap.put('AccountCode', '260');
itemMap.put('SalesDetails', salesDetailsMap);
itemMap.put('IsSold', true);
itemMap.put('Description', 'Item Description item-gold');

Map<String, Object> purchaseDetailsMap = new Map<String, Object>();
purchaseDetailsMap.put('UnitPrice', '100');
purchaseDetailsMap.put('TaxType', 'INPUT');
purchaseDetailsMap.put('COGSAccountCode', '260');
itemMap.put('PurchaseDetails', purchaseDetailsMap);
itemMap.put('IsPurchased', true);
itemMap.put('PurchaseDescription', 'Purchase Item Description - item-gold');
itemMap.put('IsTrackedAsInventory', false);
itemMap.put('QuantityOnHand', '0');
itemMap.put('TotalCostPool', '0');
itemMap.put('InventoryAssetAccountCode','630');

Map<String, Object> requestJSONMap = new Map<String, Object>();
requestJSONMap.put('items', new List<Object>{itemMap});
String reqJSON = JSON.serialize(requestJSONMap);

Map<String, Object> reqObj = new Map<String, Object>();
reqObj.put('version', '1.0');
reqObj.put('action', 'createItem');
reqObj.put('requestJSON', reqJSON);

//  Placing a request to Breadwinner Global API
Map<String, Object> respMap = Bread_winner.BreadwinnerXeroAPI.call(reqObj);
System.debug(respMap);