FILEMAKER PRO TO XERO ACCOUNTING INTEGRATION

I require my invoices that are generated in xero to be able to grab the next inv in line number from Xero accounting software and also send the invoice to Xero for accounting purposes. Xero does not allow custom fields and I require the Inv to be sent to my client in a certain format for payments.THE WORK ORDER # The CTS # + THE BUILDING NAME. I have attached a copy of the invoice format required.