Users can manually create their number series by typing the PO number in the PO#. 

1. Click on Sales > Purchase Order 
2. Click on the '+' sign to generate a new PO
3. Manually add the PO number in the Purchase Order template #, e.g. Moon PO 1
4. Enter other PO details and click on 'Save.'

After creating a PO series, all the upcoming Purchase Orders will be saved in the succeeding number—for example, Moon PO 2, Moon PO 3, and so on.