For Nagios XI is usual web access URL httphttps://some.domain:port/nagiosxi
In that case, For Nagios Core is usual web access URL is httphttps://some.domain:port/nagios and that address should be used in Nagios API URL configuration field.To check if the URL is correct, try in the web browser access http://some.domain:port/nagios/cgi-bin/objectjson.cgi?query=hostgrouplist
The actual value should replace some.domain for your Nagios installation.
The extension uses Basic authentication for Nagios JSON API, the Digest authentication is not supported by the extension.