Evaluate Import with the password-protected site in question. This will generate an authentication dialog. Enter your credentials to access the content:

• By default, Import generates a dialog for interactive authentication when establishing a network connection. See the the “ServerAuthentication” option available in Import for more details.

Use the "Username" and "Password" options available in Import to provide your credentials to the server:

• When importing a remote file, Import uses proxy and related settings specified in the Preferences settings in the notebook front end or in your operating system environment.

Use the Authentication option in URLExecute to provide your credentials to the Wolfram Cloud: