Create

//  Create a Map to store all the data of a Bill
List<Object> billList = new List<Object>();
Map<String, Object> billMap = new Map<String, Object>();
    billMap.put('Type', 'ACCPAY');
    billMap.put('DateString', '2021-11-11T00:00:00');
    billMap.put('DueDateString', '2021-12-06T00:00:00');
    billMap.put('Status', 'DRAFT');
    billMap.put('InvoiceNumber', 'This is the reference for Bill');

    //  Initializing the Bill
    Map<String, Object> contactMap = new Map<String, Object>();
    contactMap.put('ContactID', 'c84ebb15-0a8d-4145-933f-203e68d014a0');
    billMap.put('Contact', contactMap);
    //  Initializing the Bill Line Items
    List<Object> lineItemList = new List<Object>();
        Map<String, Object> lineItemMap = new Map<String, Object>(); 
        lineItemMap.put('Description', 'Line Item Description');
        lineItemMap.put('UnitAmount', 100);
        lineItemMap.put('Quantity', 10);
		lineItemMap.put('AccountCode', '310');
        lineItemList.add(lineItemMap);
        
    //  Adding the Line Items to Bill
    billMap.put('LineItems', lineItemList);
    billList.add(billMap);

//  Initializing the RequestJSON Data to be passed.
    Map<String, Object> reqJSONMap = new Map<String, Object>(); 
    reqJSONMap.put('invoices', billList); 
    String reqJSON = JSON.serialize(reqJSONMap); 

    Map<String, Object> finalReqMap = new Map<String, Object>(); 
    finalReqMap.put('version', '1.0'); 
    finalReqMap.put('action', 'createBill'); 
    finalReqMap.put('RequestJSON', reqJSON); 
    finalReqMap.put('skipDML', false); 
    //Required, if Xero Multi-Org is enabled in Breadwinner. 
    finalReqMap.put('xeroOrgId', '!aa00TM');  
    System.debug('Request ::: ' + finalReqMap);

//  Placing a request to Breadwinner Global API
    Map<String, Object> respMap = bread_winner.BreadwinnerXeroAPI.call(finalReqMap);
    System.debug('Response ::: ' + respMap);