<script type="text/javascript">
    var MyJSONText = '{"client_id":"<<MY_CLIENT_ID>>","client_secret":"<<MY_CLIENT_SECRET>>","grant_type":"client_credentials","expiration":"1440","f":"json"}';
    var MyJSON = JSON.parse(MyJSONText);
    xhr = new XMLHttpRequest();
    xhr.open("POST", "https://www.arcgis.com/sharing/rest/oauth2/token/");
    xhr.send(MyJSON);
    xhr.onreadystatechange = function ()
    {
        if (xhr.readyState == 4 && xhr.status == 200)
        {
            alert(xhr.responseText);
        }
    }

</script>