var cred= new CredentialCache();
                cred.Add(new Uri("url"), "NTLM", new NetworkCredential("user", "pwd", "***.com"));

                var hwd= new HttpClientHandler() { Credentials = credentialsCache, PreAuthenticate = true };
                
                hwd.ClientCertificateOptions = ClientCertificateOption.Manual;
                hwd.ServerCertificateCustomValidationCallback =
                    (httpRequestMessage, cert, cetChain, policyErrors) => true;
                hwd.UseDefaultCredentials = false;

                var client = new FlurlClient(new HttpClient(hwd) { Timeout = new TimeSpan(0, 0, 30) };);
                 var response = await request
                    .AllowHttpStatus(HttpStatusCode.NotFound)
                    .WithHeader("Accept", "application/json")
                    .WithClient(client)
                    .WithHeader("Content-Type", "application/json; charset=utf-8")
                    .GetAsync();

Flurl.Http.FlurlHttpException: Call failed. NTLM authentication is not possible with default credentials on this platform: 
System.PlatformNotSupportedException: NTLM authentication is not possible with default credentials on this platform. at

NAME="Alpine Linux"
ID=alpine
VERSION_ID=3.16.3
PRETTY_NAME="Alpine Linux v3.16"