For Nagios XI is usual web access URL httphttps://some.domain:port/nagiosxi
For Nagios Core is usual web access URL httphttps://some.domain:port/nagios
The actual value should replace some.domain and port should be replaced by actual values from your for your Nagios installation.
The extension is using uses Basic authentication to log infor Nagios JSON API, the Digest authentication is not supported by the extension.