contract number auto fill on the manual PO creation process

Hi Gurus,

Has anyone come across the below scenario.

"while manually creating the PO in ME21N (without any reference to PR or contract), when we enter the vendor number, plant, material number, the standard behavior of the system is that the info record number will populate on the PO. How can we make SAP auto populate the contract number on the PO when we enter the plant, vendor and material number (if we maintain the contract on the source list). Any ideas??

Thanks

Ravi