Post Sales Order from business connector with CSharp in MSD axapta

This code can help you
 AxaptaRecord AXSalesTable = ax.CreateAxaptaRecord("SalesTable");  
 AXSalesTable = ax.CallStaticRecordMethod("SalesTable", "find", SalesId) as AxaptaRecord;   
 AxaptaObject axSalesFormLetter_Invoice = ax.CreateAxaptaObject("SalesFormLetter_Invoice");  
 axSalesFormLetter_Invoice.Call("new");  
 axSalesFormLetter_Invoice.Call("update", AXSalesTable);