[jQuery] i can't modify the header with "beforeSend"

[ximiff]

I'm trying to auto login to twitter api with adding a Authorization
header. When i run my code below , i found that the request headers
still not modified with firebug, and the browser pop a window to input
username and password. How can I handle that?

<code>

var url = "http://twitter.com/account/verify_credentials.json?
callback=?";

        $.ajax({
          beforeSend: function(XMLHttpRequest) {
                XMLHttpRequest.setRequestHeader("Authorization", "Basic
eWlzdWRvbmdAZ21haWwuY29tOmV2YTBldwEs");
          },
          type: "GET",
          url: url,
          dataType: "jsonp",
          success: function(data){
                alert( "Data Saved: " + data["friends_count"] );
           }
        });


</code>