Exchange ActiveSync, using OAuth2 to read OPTIONS - oauth-2.0

Question

For an application created and configured in Azure Active Directory I am able to request and obtain a token. When I am trying to use this token to read OPTIONS from Exchange ActiveSync, the response is 401 unauthorized. The source code is below. Obviously I am doing something wrong. Any help is greatly appreciated!
Thank you!
using System.Collections.Generic;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;
namespace FAC.ActiveSync.ModernAuth
{
internal partial class Program
{
private static readonly AzureApp app = new AzureApp
{
TenantId = "***",
ClientId = "***",
ClientSecretId = "***",
ClientSecretValue = "***"
};
static async Task Main()
{
var response = await GetOptions();
}
private static async Task<AccessTokenModel> GetToken()
{
var data = new Dictionary<string, string>
{
{ "grant_type", "client_credentials"},
{ "scope", "https://outlook.office365.com/.default"},
{ "client_id", app.ClientId},
{ "client_secret", app.ClientSecretValue}
};
using (var client = new HttpClient())
{
HttpResponseMessage response =
await client.PostAsync(app.UrlGetToken,
new FormUrlEncodedContent(data));
return await response.Content.ReadAsAsync<AccessTokenModel>();
}
}
private static async Task<string> GetOptions()
{
var accessToken = await GetToken();
var request = new HttpRequestMessage(
HttpMethod.Options,
"https://outlook.office365.com/Microsoft-Server-ActiveSync");
using (var client = new HttpClient())
{
client.DefaultRequestHeaders.Authorization =
new AuthenticationHeaderValue("Bearer",
accessToken.access_token);
var response = await client.SendAsync(request);
return await response.Content.ReadAsStringAsync();
}
}
}
public class AccessTokenModel
{
public string access_token { get; set; }
public string token_type { get; set; }
public int expires_in { get; set; }
public int ext_expires_in { get; set; }
}
public class AzureApp
{
public string TenantId { get; set; }
public string ClientId { get; set; }
public string ClientSecretValue { get; set; }
public string ClientSecretId { get; set; }
public string UrlGetToken
{
get
{
return $"https://login.microsoftonline.com/{TenantId}/oauth2/v2.0/token";
}
}
public string UrlAuthorize
{
get
{
return $"https://login.microsoftonline.com/{TenantId}/oauth2/v2.0/authorize";
}
}
}
}