Most sites will have a on-site authentication system like this, which will use a form or API to authenticate the user:
Basic access authentication
The browser itself will prompt for a username and password and will look like this:
Normally you'll have something like this if it's a staging environment, an internal application or the application is still in development. To get the site to work with Nickelled you should set the username and password in the 'Advanced configuration' of the guide creation.