On completion, you can access your nagios instance at:
https://yourdomain.com/nagios
The default user name is:
nagiosmin
A unique password for HTTP access is created and saved to:
/etc/webmin/nagios/aut.txt
Example contents for auth.txt are as below:
nagiosmin http pass:sTpoL-dBhQ
Note
For security purposes, you change the default admin name as well as the nagios url.