BotAuthentication.cs

using(var httpClient = new HttpClient())
{
  // Set headers
  httpClient.Timeout = new TimeSpan(0, 2, 0);  // 2 minutes
  httpClient.BaseAddress = new Uri(_serviceUrl);
  httpClient.DefaultRequestHeaders.Add("OData-MaxVersion", "4.0");
  httpClient.DefaultRequestHeaders.Accept.Add(
      new MediaTypeWithQualityHeaderValue("application/json"));
  // Use the bearer token here
  httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", tokenResponse.Token);

  HttpResponseMessage whoAmIResponse = await httpClient.GetAsync("api/data/v9.1/WhoAmI()");
  Guid userId;
  if (whoAmIResponse.IsSuccessStatusCode)
  {
      var jWhoAmIResponse = JObject.Parse(whoAmIResponse.Content.ReadAsStringAsync().Result);

      // Finally, Get the userId
      userId = (Guid)jWhoAmIResponse["UserId"];
  }
}