Hi Madhan,
In SRM at business partner level you first need to set the output medium to XML/XI and then in SPRO you can set the method call for XML for the POs as shown in attachment.
Please make sure the SAP PI has got the correct configurations in place. The vendor URL to which this output need to be sent is configured within SAP PI this is based on the vendor number. You can then make sure that the output is going out of SRM to PI using tcode SXMB_MONI.
The output can go to PI and through PI, it can reach to the vendor destination.
Regards,
Ravi Pachauri