var messageText = 'Тут текст сообщения',
    http = new XMLHttpRequest(),
    url = 'https://api.carrotquest.io/v1/users/' + carrotquest.data.user.id + '/startconversation',
    params = 'auth_token=' + carrotquest.data.auth_token + '&id_as_string=true&body='+messageText;
http.open('POST', url, true);
http.setRequestHeader('Content-type', 'application/x-www-form-urlencoded; charset=UTF-8');
http.send(params);
http.onreadystatechange = function() {
    if (http.readyState != 4) return;
    if (http.status == 200) {
        var json = JSON.parse(http.responseText);
        carrotquest.messenger.toStateNo();
        carrotquest.messenger.toStateOpened({id: json.data.id});
    };
};