HomeController.cs

public class HomeController : Controller
    {
        private static string clientId = ConfigurationManager.AppSettings["ida:ClientId"]; //clientId
        private static string appKey = ConfigurationManager.AppSettings["ida:AppKey"]; //clientSecret
        private static string aadInstance = ConfigurationManager.AppSettings["ida:AADInstance"]; //https://login.windows.net/{0}
        private static string tenant = ConfigurationManager.AppSettings["ida:Tenant"]; //TenantName.onmicrosoft.com
        private const string ResourceUri = "https://onenote.com";
        public static readonly string Authority = String.Format(CultureInfo.InvariantCulture, aadInstance, tenant);//https://login.windows.net/TenantName.onmicrosoft.com
        Uri returnUri = new Uri("https://localhost:44327/Home/ReturnFlow");

        public ActionResult Index()
        {
            string authorizationUrl = string.Format(
               aadInstance + "/oauth2/authorize?api-version=1.0&response_type=code&client_id={1}&resource={2}&redirect_uri={3}",
                   tenant,
                   clientId,
                   ResourceUri,
                   returnUri.AbsoluteUri
               );
               
               /*
              https://login.windows.net/TenantName.onmicrosoft.com/oauth2/authorize?api-version=1.0&response_type=code
                &client_id=9eaffd31-8592-4c96-892c-32adb9a432b8&resource=https://onenote.com&redirect_uri=https://localhost:44327/Home/ReturnFlow
             */

            return new RedirectResult(authorizationUrl);
        }

        public async Task<ActionResult> ReturnFlow(string code)
        {
            AuthenticationContext AuthContext = new AuthenticationContext(Authority);
            ClientCredential credential = new ClientCredential(clientId, appKey);
            var result = await AuthContext.AcquireTokenByAuthorizationCodeAsync(code, returnUri, credential);

            HttpClient client = new HttpClient();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", result.AccessToken);
            string apiRoute = "https://www.onenote.com/api/beta/me/notes/notebooks";
            var getMessage = new HttpRequestMessage(HttpMethod.Get, apiRoute);
            HttpResponseMessage response = await client.SendAsync(getMessage);
            string data = await response.Content.ReadAsStringAsync();

            return Json(data, JsonRequestBehavior.AllowGet);
        }
    }