c#, .net.websockets - ClientCertificate

Question

i have a websocket client and server and i'm trying to add a certificate to my client, but i don't think i have the hang of it yet..

i have an asp.net core 7 web api and wonder what i do wrong

Client:

public async Task<string> GetEcho(string echoMessageKey, string echoMessageValue)
        {
X509Certificate2 certificate = new X509Certificate2("certPath", "password");

            _clientWebSocket.Options.ClientCertificates.Add(certificate);

 //connect to server
            var xferURI = new Uri(uri);
            await _clientWebSocket.ConnectAsync(xferURI, CancellationToken.None);
           

            var messageInEcho = new Dictionary<string, string>
            {
                { echoMessageKey,echoMessageValue }
            };
            //specify command
            var echoCommand = new
            {
                echo = messageInEcho
            };
            return await ReceiveMessage(echoCommand);
        }


public async Task<string> ReceiveMessage(object? Command)
        {
            var commandJSON = Encoding.UTF8.GetBytes
                                    (JsonConvert.SerializeObject(Command));
            await _clientWebSocket.SendAsync(new ArraySegment<byte>(commandJSON)
                                            , WebSocketMessageType.Text, true, CancellationToken.None);

            // receive and process the websocket message
            var buffer = new byte[1024 * 60];//new byte[1024];
            var result = await _clientWebSocket.ReceiveAsync(new ArraySegment<byte>(buffer),
                        CancellationToken.None);
            var message = Encoding.UTF8.GetString(buffer, 0, result.Count);

            // close connect to websocket
            await _clientWebSocket.CloseAsync(WebSocketCloseStatus.NormalClosure,
                                            "", CancellationToken.None);

            return message;
}

[HttpGet("GetEcho")]
        [ActionName(nameof(GetEcho))]
        public async Task<IActionResult> GetEcho(string echoMessageKey, string echoMessageValue)
        {
            try
            {
                var echo = await _yunexXFER.GetEcho(echoMessageKey, echoMessageValue);
                return Ok(echo);
            }
            catch (Exception ex)
            {
                return StatusCode(500, ex.Message);
            }
        }

Server:

var wsOptions = new WebSocketOptions { KeepAliveInterval = TimeSpan.FromSeconds(120) };
app.UseWebSockets(wsOptions);

app.Use(async (context, next) =>
        {
            if (context.Request.Path == "/test")  
            {
                /*TEST*/
                if(context.Connection.ClientCertificate!=null) //alwayse null, why?
                { }
                /*TEST*/
                if (context.WebSockets.IsWebSocketRequest)
                {

                    using var webSocket = await context.WebSockets.AcceptWebSocketAsync();
                    await Echo(webSocket);
                }
                else
                {
                    context.Response.StatusCode = StatusCodes.Status400BadRequest;
                }
            }
            else
            {
                await next(context);
            }
        });

i think i do somethink here wrong ?

X509Certificate2 certificate = new X509Certificate2("certPath", "password");

_clientWebSocket.Options.ClientCertificates.Add(certificate);

it could also be on the server that I maybe still have to specify something for the certificate, so much knowledge I have then unfortunately also not of

edit - I test the whole thing with swagger