AuthHelper.cs

using Newtonsoft.Json;
public class AuthHelper
{
    /// <summary>
    /// The header to use for OAuth authentication.
    /// </summary>
    public const string OAuthHeader = "Authorization";
    private HttpClient client = new HttpClient();
    public ClientConfiguration clientConfiguration { get; set; }

    /// <summary>
    /// Retrieves an authentication header from the service.
    /// </summary>
    /// <returns>The authentication header for the Web API call.</returns>

    public async Task<string> getToken()
    {
        Dictionary<string, string> postData = new Dictionary<string, string>();
        postData.Add("grant_type", "client_credentials");
        postData.Add("client_id", clientConfiguration.ClientId);
        postData.Add("client_secret", clientConfiguration.ClientSecret);
        postData.Add("resource", clientConfiguration.Resource);

        string url = $"https://login.microsoftonline.com/{clientConfiguration.TenantId}/oauth2/token";

        string result = string.Empty;

        try
        {
            string response = await PostHTTPRequestAsync(url, postData);
            AuthResponse authresponse = JsonConvert.DeserializeObject<AuthResponse>(response);

            result = authresponse.access_token;

        }
        catch (Exception ex)
        {
            //ConsoleList.(ex.Message);
        }

        return result;
    }


    private async Task<string> PostHTTPRequestAsync(string url, Dictionary<string, string> data)
    {
        HttpContent formContent = new FormUrlEncodedContent(data);
        HttpResponseMessage response = await client.PostAsync(url, formContent).ConfigureAwait(false);
        response.EnsureSuccessStatusCode();
        return await response.Content.ReadAsStringAsync().ConfigureAwait(false);
    }

 
 
}

public class AuthResponse

{
    public string token_type { get; set; }
    public string expires_in { get; set; }
    public string ext_expires_in { get; set; }
    public string expires_on { get; set; }
    public string not_before { get; set; }
    public string resource { get; set; }
    public string access_token { get; set; }
}