How to generate PO requisitions from Discrete Job

We have a BOM to be assembled.  In that BOM all the items are purchase items.  Once the items are procured on a PO they will be assembled into a finished good using a discrete job.

We don't maintain on-hand of these items, we want to buy on the fly when we got the order from the customer.

How to generate PO Requisitions from Discrete Job if on-hand quantity is less than quantity required for the job?