Trigger IDoc whenever new PR is created

Hi,

I have a requirement where I would like to generate an IDOC whenever a new  Purchase Requisition (PR) is created.

                 Standard basic type :  PREQCR01

                 Message type is       :   PREQCR

But i came to know that output determination is not available for ME51N Transaction.

So how can trigger IDoc.??

Thank You