Purchase Order Numbers

Hi there, how can I please tell zohoCRM where to start from issuing a PO Number please? I can't edit the field "PO Number" that's in the module however if I try to add a new field (I've called in P/O Number to differentiate), made this a recurring number with a prefix and a number to start from, and added to the top section of the PO - It doesn't actually show up in the purchase order at all

Thanks

Mike