xmlHttpRequest.Send() issues.

[TimeWarDoctor]

function getResponse() {



        var currentDate = new Date();
        var sendMessage = JSON.stringify({
            SendTimestamp: currentDate,
            Message: "Message 1"
        });

$.ajax({
            type: "POST",
            url: "http://L45723:1802/?callback=testcallback",
            data: sendMessage,
            contentType: "application/json",
            beforeSend: function(xhr) {
                 xhr.setRequestHeader("Authentication", "Basic " + $.encodeBase64("server:server123")); //May need to use "Authorization" instead
            },
            success: function(data){
                $('#_receivedMsgLabel').append(data.Message + '<br/>');
                getResponse();
            }
        });

    }

Changed my code to as above...as in the link, but don't think I understood it because I got this in Firefox:

$.encodeBase64 is not a function
beforeSend(xhr=Object { readyState=0, setRequestHeader=function(), getAllResponseHeaders=function(), more...})ShowComet9.aspx (line 39)
ajax(url=undefined, options=Object { type="POST", url="http://L45723:1802/?callback=testcallback", data="{"SendTimestamp":"2012-...,"Message":"Message 1"}", more...})jquery-1.7.1.js (line 7547)
getResponse()ShowComet9.aspx (line 41)
()ShowComet9.aspx (line 56)
[Break On This Error]

...thentication", "Basic " + $.encodeBase64("server:server123")); //May need to use...

Where do I get the encodeBase64 jQuery function from?