PunchOut is not a link to a web site. PunchOut is an easy-to-implement protocol for interactive sessions managed across the Internet. PunchOut is a communication from one application to another, achieved through a dialog of real-time, synchronous cXML messages, which support user interaction at a remote site. It is the best way to manage user sessions distributed across web applications.
To configure PunchOut for a vendor simply go to Vendor Info - PunchOut SetupVendor Maintenance and enter the proper PunchOut settings.