Refactor code for API wrapper
Well, you should refactor your code so that you don't repeat yourself:

private async Task&lt;List&lt;ItemRetrieved&gt;&gt; GetValuesAsync(object parametersAsAnonymousObject)
{
    var requestUrl = MakeRequestUri("v3/get");

    var request = new HttpRequest(HttpMethod.Post, requestUrl);
    request.AddBody(parametersAsAnonymousObject);

    var response = await SendAsync&lt;ItemRetrievedWrapper&gt;(request);
    return response.List.Values.ToList();
}

public Task&lt;List&lt;ItemRetrieved&gt;&gt; GetAllUnreadAsync(int sinceInUnixTime = 0)
{
    return GetValuesAsync(new
        {
            consumer_key = _consumerKey,
            access_token = _accessToken,
            state = "unread",
            sort = "newest",
            since = sinceInUnixTime
        });
}

public Task&lt;List&lt;ItemRetrieved&gt;&gt; GetFavoriteAsync(int sinceInUnixTime = 0)
{
    return GetValuesAsync(new
        {
            consumer_key = _consumerKey,
            access_token = _accessToken,
            favorite = 1,
            sort = "newest",
            since = sinceInUnixTime
        });
}


This sill repeats consumer_key and access_token, but I think getting rid of that repetition would be too complicated, so it's not worth it (unless you have big number of these methods).