In Pitstop activity the Pitstop.Credentials.Url property is not available for binding. However, it is possible to set the Credentials using the following syntax: wf_TestPitStop.xml

If not explicitly configured into the workflow, then the Pitstop activity reads the GN4 security credentials. So you can associate the credentials to a specific url into the User/Group editing dialog, and the Pitstop activity will use that credentials when accessing to that url.