is ajax post with basic http authentication possible using jquery?

I have a code which tries to post to a api. But I keep getting 401 in firefox and 400 in safari

      $.ajax({
        type: "POST",
        url: "http://url.com",
        dataType: "json",
        async: "false",
        data: form_data,
        beforeSend: function(xhr1) {
          xhr1.setRequestHeader("Authorization", "Basic " + encodeBase64(username + ":" + password))
        },
        success: function (data,status,xhr){
          //do something
        }
      });

I followed instructions from here
http://dothow.blogspot.com/2009/05/http-basic-authentication-with-jquery.html?showComment=1270999665472#c3021624182011440325

*encodeBase64 is part of a library function