This feature of Checkbox/Ultimate Survey currently relies on the web server to perform the authentication. When IIS authenticates a user, it sets a Server Variable that Checkbox can read from.
To enable this in Checkbox Web, you need to edit the IIS configuration for your site and enable Integrated Windows Authentication. Next, you need to log-in to Checkbox and go to Settings->Network Users and enable network user login. When the network authentication is enabled, a network user only needs to go to Default.aspx rather than Login.aspx and they will automatically be logged-in to Checkbox Web.
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