There are two types of authentication mechanisms that CrossBrowserTesting supports for screenshots: HTML Login Forms and Basic/HTTP Authentication. It is recommended to understand the difference in order to achieve the intended results on Screenshot Tests.
HTML login forms
This is the standard page you get when you load a site in the browser and it sends you to an HTML form on the website itself for authenticating.
For a step-by-step guide to create login profiles for HTML form-based logins, see Working With a Login Profile.
Click the image to enlarge it.
Basic and HTTP authentication
This is where the browser will not even load the web page at all but instead prompts for the username/password with its own pop-up window.
To run screenshots on a site protected by Basic authentication, see Advanced Options on the new screenshot page and enter the username and password required.
Click the image to enlarge it.
For more information on issues that may occur, see Troubleshooting Screenshots and scroll to Login Forms or Basic Authentication.
Have pages behind a login that has more fields than just a user name and password? You can also record a Selenium script and use it to navigate the screenshot system to a particular page: Can I use a Selenium script with the screenshot system?