How can add the Customer PO ref in Statement of account
Put it into the transaction-level Description field of the sales invoice. It will show up as the description of the transaction. Otherwise, you cannot, because a customer purchase order is not a transaction that appears in Manager.
However, your sales order number (if you are using the Sales Orders tab) will show up as a column. If you enter customer purchase order numbers as your sales order numbers, they will appear there.