There are two ways you can address this situation.
1) If you search on the PO number in the search field you will get a return of the order that is associated with it. See images below. First I entered the PO number in my search field and it returned my order that used that PO. The second image shows the order detail so you can see that the order it returned did in fact use the PO number I searched for.
2) The other option if you are not integrating with a 3rd party solution, or they are not currently using the Job # Field to return a Job # for the order, you can edit the order when it comes in and add the Job # to the item. Then it will appear in your order grid for easy filtering. See images below. I went into the order detail for my order item and added the PO number in the Job# field. The second image shows that once returning to the order summary grid, I can see that my PO number is now showing in the Job # column that I can now filter on.