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 installation for your Nagios installation.
The extension uses Basic authentication for Nagios JSON API, the Digest authentication is not supported by the extension.