As soon as a Purchase Order that you created is fully approved and sent to Xero, its Purchase Order number is assigned automatically. These numbers ascend incrementally. So, for example, if you have just started using Xero, your first Purchase Order will automatically receive the number PO-001.
Please note: if you have deleted a Purchase Order, the system will find that gap and assign the respective number to the next Purchase Order you create. For example, you have PO-046, PO-047, PO-048 and PO-049. Then you delete PO-047 permanently. This means, the next Purchase Order you create will receive the number PO-047 instead of PO-050.