if you turn on the integration between WO and PO, individual lines on a PO can be linked to a work order.
A couple of things to consider:
The work order has to be released before costs from PO can be posted to the work order
Costs of inventory items are posted to the work order during receipt of GOODS entry.
Costs of Miscellaneous items and Miscellaneous Charges are posted to the work order during receipt of INVOICE (or when an invoice number is used in ROG)
if the items are in a BOM or manually entered on the work order, you can set the issue method to automatic and the items that they sent out for assembly will be taken from stock as soon as the work order is released, (or you could do a manual issue )