If you check the Manual Numbers field to true for the number series assigned to Purchase Orders then you will be able to see the No. field on the Purchase Order.
However, like Ben said, this is not a good idea to do. Its just not good for an audit trail, its better if you have multiple automatic number series you can select from. It also makes the Purchase Order entry process slower, every time a user needs to come up with a number and ensure its unique and hasnt been used before. What's the use case that you need the number series to be manual?
You have to make an Extension to make the PO No. field an option to add to the Page via Personalization. Once the field is on the Page, as long as your No. Series has "Manual" allowed you'll be able to type into the No. field. This would be the same approach if you wanted to have multiple No. Series to select from, which is the recommended approach over Manual entry.