This works, and I get token.

Update: As @stefen pointed out that: RFC 7231 4.3.1:

"A payload within a GET request message has no defined semantics; sending a payload body on a GET request might cause some existing implementations to reject the request."

I will leave example implementation of given call.

My HttpWrapper.cs:

public static class HttpClientWrapper<T> where T : class
    {
        public static async Task<T> GetUrlEncoded(HttpClient httpClient, string url, Dictionary<string, string> parameters)
        {
            T result = null;
            var request = new HttpRequestMessage(HttpMethod.Get, url) { Content = new FormUrlEncodedContent(parameters) };
            var response = await httpClient.SendAsync(request).ConfigureAwait(false);

            response.EnsureSuccessStatusCode();
            await response.Content.ReadAsStringAsync().ContinueWith((Task<string> x) =>
            {
                if (x.IsFaulted)
                    throw x.Exception;

                result = JsonConvert.DeserializeObject<T>(x.Result);
            });

            return result;
        }
    }

Usage of Http wrapper and Http call as so:

            Dictionary<string, string> urlParams = new Dictionary<string, string>();
            urlParams.Add("username", "Username");
            urlParams.Add("password", "Password");
            urlParams.Add("grant_type", "password");

            AuthResultViewModel outcome = await HttpClientWrapper<AuthResultViewModel>.GetUrlEncoded(httpClient, "https://some-endpoint.com/token", urlParams);