We need to personalize the contract purchase agreement form

Hi,

We are working to implement the 'Contract Purchase Agreement' to have tracking and reporting on POs referred by a baseline contract. There is business requirement to embed an alert or warning message on 'Standard Purchase Order' saying 'Valid Contracts are available for this supplier' to avoid that buyer may still create a 'Standard Purchase Order' by over looking the contract made for the supplier in the 'Standard Purchase Order'. How can we achieve the objective?