It must be a Wagon URL like the URL used in distributionManagement. For the Webdav protocol, you can use dav:[http_url], or file:///path_to_the_directory if you want to copy it locally with the file protocol.
<server> <username>admin</username> <password>admin123</password> <id>continuum.site</id> </server>
Using the server configuration in [user_home]/.m2/settings.xml is a workaround for user authentication using the Wagon notifier.
The <username> and <password> should be set to the user account that has the Site Producer and Site Observer roles.
The file protocol will create a buildresult.txt file locally in the location specified in the Wagon Notifier configuration. If you use the Webdav protocol, a buildresult.txt file will be copied to your disk, relative to where you told it to be placed when you set up the Wagon notifier.
You can also view the buildresult.txt file from a browser at the Project Site URL you specified.