Give the url to check.
You are able to give a template value (a value containing some environment variables to solve).
By default, environment variables used by polling mechanism are supplied by the last build.
For the first build, Jenkins tries to resolve the environment variables from the current polling node.
Be careful: in the context of a previous build with no environment variables and a value with node or job environment variables, it is possible to have a build without a real URL change (due to a wrong URL check at Jenkins startup when all nodes are not up.)