"InvoiceSourceSystem": "APTTUS",
    "ExternalCompanyId": "APTTUS",
    "FromDate": "2019-08-01",
    "ToDate": "2019-08-03",
    "TotalNumberOfInvoices": "109",
    "Invoices": [
        {
            "HeaderKey": "1",
            "AddOnly": "",
            "AutoComplete": "Y",
            "CustomerInvoiceID": "378958d7-d4af-e911-bcd0-00155d69e630",
            "Submit": "Y",
            "LockedinWorkday": "N",
            "Company": "5103",
            "Currency": "USD",
            "Customer": "EQ145959",
            "SoldToCustomer": "",
            "BillToContact": "",
            "BillToAddress": "",
            "RowID": "",
            "InvoiceNumber": "INV-000000003531",
            "InvoiceDate": "2019-07-26",
            "AccountingDate": "",
            "FromDate": "",
            "ToDate": "",
            "DueDateOverride": "2019-08-25",
            "ControlTotalAmount": "",
            "PaymentTerms": "",
            "PaymentType": "",
            "AmountDue": "20663.82",
            "IncludeCustomerWorktags": "",
            "Memo": "",
            "InvoiceLineItem": [
                {
                    "HeaderKey": "1",
                    "LineKey": "1",
                    "CustomerInvoiceLine": "",
                    "CustomerInvoiceLineReferenceID": "INV-000000003531-1",
                    "LineOrder": "",
                    "IntercompanyAffiliate": "",
                    "SalesItemIDType": "",
                    "SalesItemIDValue": "",
                    "RevenueCategory": "RC900",
                    "LineItemDescription": "Invoice total",
                    "ShipToCustomerIDType": "",
                    "ShipToCustomerIDValue": "",
                    "ShipToAddressIDType": "",
                    "ShipToAddressIDValue": "",
                    "Quantity": 1,
                    "UnitofMeasure": "EA",
                    "UnitCost": "",
                    "ExtendedAmount": "20663.82",
                    "TransactionDate": "",
                    "FromDate": "",
                    "ToDate": "",
                    "DeferredRevenue": "",
                    "RevenueRecognitionScheduleTemplate": "",
                    "Memo": "",
                    "AnalyticalAmount": "",
                    "Product": "",
                    "IssueNumber": "",
                    "IssueNumberType": "",
                    "CostCenter": ""
                },


My Code:

/*

 */
@RestResource(urlMapping='/WorkdayIntegration/') 
global class APTS_InvoiceDetailsWS
{

@HttpPost
    global static List<APTS_WorkdayIntegrationWrapper> getInvoiceDetails()
    { 
        
        String JsonString;
        RestRequest req = RestContext.request;        
        RestResponse res = RestContext.response;
         Map<String, Object> params = (Map<String, Object>)JSON.deserializeUntyped(RestContext.request.requestBody.toString());
        Date startDate=Date.valueOf((String)params.get('StartDate'));
        Date endDate=Date.valueOf((String)params.get('EndDate'));
        String approved='Approved';
        String query='Select id,Name,EQ_InvoiceApprovedDate__c,Apttus_billing__status__c,CurrencyIsoCode,Apttus_Billing__ShipToAccountId__r.EQ_ClientRef__c,Apttus_Billing__ShipToAccountId__r.EQ_ShippingState__c,Apttus_Billing__InvoiceDate__c,EQ_DueDateCustom__c,EQ_TotalTaxAmount__c from Apttus_Billing__Invoice__c where EQ_InvoiceApprovedDate__c>=:startDate and EQ_InvoiceApprovedDate__c<=:endDate and EQ_TotalInvoiceAmount__c > 0 and Apttus_billing__status__c=:approved'; 
            
        List<Apttus_Billing__Invoice__c> invoiceObjectList1=Database.query(query);
        List<APTS_WorkdayIntegrationWrapper> lstInvoiceHistory = new List<APTS_WorkdayIntegrationWrapper>();
        Integer count=0; 
      for(Apttus_Billing__Invoice__c invoice:invoiceObjectList1)
              {
               count=count+1;
               APTS_WorkdayIntegrationWrapper invoiceWrapper=new APTS_WorkdayIntegrationWrapper();
                invoiceWrapper.InvoiceSourceSystem='APTTUS';
                invoiceWrapper.ExternalCompanyId='APTTUS';
                invoiceWrapper.FromDate=(String)params.get('StartDate');
                invoiceWrapper.ToDate=(String)params.get('EndDate');
                invoiceWrapper.TotalNumberOfInvoices=count;
                invoiceWrapper.HeaderKey=null;
                invoiceWrapper.AddOnly='';
                invoiceWrapper.AutoComplete='Y';
                invoiceWrapper.CustomerInvoiceID=invoice.id;
                invoiceWrapper.Submit='Y';
                invoiceWrapper.LockedinWorkday='N';
                invoiceWrapper.Company='5103';
                invoiceWrapper.CurrencyISOCode=invoice.CurrencyIsoCode;
                invoiceWrapper.Customer=''; /* need to confirm accountr*/
                invoiceWrapper.SoldToCustomer='';
                invoiceWrapper.BillToContact='';
                invoiceWrapper.BillToAddress='';
                invoiceWrapper.RowID='';
                invoiceWrapper.InvoiceNumber=invoice.Name;
                invoiceWrapper.InvoiceDate=invoice.Apttus_Billing__InvoiceDate__c;
                invoiceWrapper.AccountingDate='';
                //invoiceWrapper.FromDate
                //invoiceWrapper.ToDate
                invoiceWrapper.DueDateOverride='';
                invoiceWrapper.ControlTotalAmount=invoice.EQ_TotalTaxAmount__c;
                invoiceWrapper.PaymentTerms='';
                invoiceWrapper.PaymentType='';
                invoiceWrapper.AmountDue='';
                invoiceWrapper.IncludeCustomerWorktags='';
                invoiceWrapper.Memo='';
                lstInvoiceHistory.add(invoiceWrapper);
             }
        return lstInvoiceHistory;
    }
    
    
    global class APTS_WorkdayIntegrationWrapper
    {
    
        public String InvoiceSourceSystem;
        public String ExternalCompanyId;
        public integer TotalNumberOfInvoices;
        public DecimAL HeaderKey;        
        public String AddOnly;
        public String AutoComplete;
        public String CustomerInvoiceID;
        public String Submit;
        public String LockedinWorkday;
       // public List<Invoices> Invoices;
        public String Company;
        public String CurrencyISOCode;
        public String Customer;
        public String SoldToCustomer;
        public String BillToContact;
        public String BillToAddress;
        public String RowID;
        public String InvoiceNumber;
        public DateTime InvoiceDate;
        public String AccountingDate;
        public String FromDate;
        public String ToDate;
        public String DueDateOverride;
        public Decimal ControlTotalAmount;
        public String PaymentTerms;
        public String PaymentType;
        public String AmountDue;
        public String IncludeCustomerWorktags;
        public String Memo;
        
      
    } 
     public class fromJSON
    {
        public String Parm1;    //Value1
        public String Parm2;    //Value2
    }
      
}