Unexpected status code returned from negotiate: 404 Not Found - Getting it while handshaking with SignalR from Client side Android

Question

I am doing a POC where I need to establish one connection with SignalR and subscribe for notification. But while hitting the 1st negotiate api itself I am getting 404 error, but the thing is I am getting success in Postman for same url with some custom headers.

SignalRClient.java

    import android.util.Log;
    import com.microsoft.signalr.HubConnection;
    import com.microsoft.signalr.HubConnectionBuilder;
    import com.microsoft.signalr.HubConnectionState;
    import com.microsoft.signalr.OnClosedCallback;
    import com.microsoft.signalr.TransportEnum;

    import java.util.HashMap;
    import java.util.Map;

    public class SignalRClient {
    private static final String TAG = "xxxxxxx";
    private static final String HUB_URL = "xxxxxxx";

    private final HubConnection hubConnection;
    public SignalRClient(String token, String username) {
        Map<String, String> headers = new HashMap<>();
        headers.put("token", token);
        headers.put("userName", username);
        
        // Build the connection
        hubConnection = HubConnectionBuilder
                .create(HUB_URL).withHeaders(headers)
                .withTransport(TransportEnum.LONG_POLLING)
                .build();
    }

    public void startConnection() {
        if (hubConnection != null && hubConnection.getConnectionState() == HubConnectionState.DISCONNECTED) {
            hubConnection.start()
                    .blockingAwait(); // You may handle the result asynchronously

            Log.d(TAG, "Connection started");
        }
    }

    public void stopConnection() {
        if (hubConnection != null && hubConnection.getConnectionState() == HubConnectionState.CONNECTED) {
            hubConnection.stop();

            Log.d(TAG, "Connection stopped");
        }
    }
    
    public void sendMessage(String message) {
        if (hubConnection != null && hubConnection.getConnectionState() == HubConnectionState.CONNECTED) {
            hubConnection.send("SendMessage", message);

            Log.d(TAG, "Message sent: " + message);
        } else {
            Log.e(TAG, "Connection not established or closed. Unable to send message.");
        }
    }
}

I am just initializing SignalRClient from different class and start connection after that. Please guide me if I am done something wrong.