Purchase Order (PO) Inbound with external number range

We need to create a interface for Purchase order Inbound.

The PO will be created in 3rd party system, and the information is sent thro IDoc(Orders05). How do we create a PO in SAP with the same PO number as in 3rd party system.

We have allowed PO with external number range.

I couldnt find a Segment or field where the PO number can be mapped in the IDOC.

Points will be offered for useful informations.

Thanks,

Sivaraman