If the target page is using Windows authentication it's easy:
WebClient MyWebClient = new WebClient();
MyWebClient.Credentials = new NetworkCredential("myusername","mypassword");
Byte[] PageHTMLBytes;
...
This will add the windows credentials into the authentication header.
If a different authentication mechanism is being used, you will need to provide the credentials in a different way. A really good example for forms authentication can be found at:
c# - WebClient accessing page with credentials - Stack Overflow[
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