using (var client = new HttpClient())
{
    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
    "Basic", Convert.ToBase64String(Encoding.ASCII.GetBytes($"{client_id}:{client_secret}")));

    var data = new FormUrlEncodedContent(new[]
    {
        new KeyValuePair<string, string>("grant_type", "client_credentials")
    });

    var resp = await client.PostAsync("https://us.battle.net/oauth/token", data);
    resp.EnsureSuccessStatusCode();
    var result = await resp.Content.ReadAsStringAsync();
}