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);