var url = "https://api.outpan.com/v1/products/0796435419035";
var xmlhttp = new XMLHttpRequest();
var encodedString = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
xmlhttp.open("GET", url, true);

xmlhttp.setRequestHeader("Authorization", "Basic" + encodedString);
xmlhttp.send();
var temp = xmlhttp.responseXML;

{
    "error": {
        "code": 1000,
        "message": "Authentication is required."
    }
}

Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

xmlhttp.setRequestHeader("Authorization", "Basic " + encodedString);

var temp = xmlhttp.responseXML;

xmlhttp.onreadystatechange = function () {
    if (xmlhttp.readyState === 4 && xmlhttp.status === 200) {
        var response = xmlhttp.responseText;
        console.log(response);
    }
};