Sales Line Reservation from Purchase Line

Hi,

I am setting up reservations and currently I have a sales order with quantity 4 of Item ABC. I would like to reserve this item from an inbound purchase order i.e. I have made a purchase order with the same Item ABC (PO is still not received or invoiced). In the sales order, when I try to reserve this item, I am only able to reserve from the Item Ledger Entries and there is no option to reserve from the purchase lines of the PO.

Am I missing something, please?

Thanks in advance,
Y_1716