This should be an option on the Purchase Order, please go through the Cog Wheel and go to Account and Settings from here you should then see an Expenses option from here under Purchase Orders you should be able to see the option for Custom Transaction Numbers.
You should then have the customer option on the Purchase Order please let us know if you are still unable to see this particular option.
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You can change the transaction number on purchase orders. When opening a purchase order, there should be a box on the right hand side labelled PO no. which you can then edit.
This should be an option on the Purchase Order, please go through the Cog Wheel and go to Account and Settings from here you should then see an Expenses option from here under Purchase Orders you should be able to see the option for Custom Transaction Numbers.
You should then have the customer option on the Purchase Order please let us know if you are still unable to see this particular option.