using (HttpClient client = new HttpClient())
{
  var tokenEndpoint = @"https://login.windows.net/<tenant-id>/oauth2/token";
  var accept = "application/json";

  client.DefaultRequestHeaders.Add("Accept", accept);
  string postBody = @"resource=https%3A%2F%2Fgraph.microsoft.com%2F
  &client_id=<client id>
  &grant_type=password
  &username=xxx@xxx.onmicrosoft.com
  &password=<password>
  &scope=openid";

  using (var response = await client.PostAsync(tokenEndpoint, new StringContent(postBody, Encoding.UTF8, "application/x-www-form-urlencoded")))
  {
    if (response.IsSuccessStatusCode)
    {
      var jsonresult = JObject.Parse(await response.Content.ReadAsStringAsync());
      token = (string)jsonresult["access_token"];
    }
  }
}