serviceCollection
    .AddAuthentication(o =>
    {
        o.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
        o.DefaultChallengeScheme = OpenIdConnectDefaults.AuthenticationScheme;
    })
    .AddCookie(CookieAuthenticationDefaults.AuthenticationScheme)
    .AddMicrosoftAccount("Microsoft", "Microsoft", o =>
    {
        o.SignInScheme = IdentityConstants.ExternalScheme;
        o.ClientId = _externalKeysOptions.MicrosoftClientId;
        o.ClientSecret = _externalKeysOptions.MicrosoftClientSecret;
        o.CallbackPath = new PathString("/signin-microsoft");
        new[]
        {
            "offline_access",
            "Calendars.Read.Shared",
            "Calendars.ReadWrite",
            "Tasks.Readwrite"
        }.ForEach(scope => o.Scope.Add(scope));
        o.SaveTokens = true;
    })

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
[Route("externallogin")]
public IActionResult ExternalLogin(ExternalLoginModel model)
{
    var redirectUrl = Url.Action("ExternalLoginCallback", "Account", new { model.ReturnUrl, termsOfServiceAccepted = model.AgreeToTerms, platform = model.Platform });
    var properties = _signInManager.ConfigureExternalAuthenticationProperties(model.Provider, redirectUrl);

    return Challenge(properties, model.Provider);
}