client.TcpHandler = (address, port) =>
{
    var proxy = new Socks5ProxyClient(
        proxyHost, proxyPort, proxyUsername, proxyPassword);
    return System.Threading.Tasks.Task.FromResult(
        proxy.CreateConnection(address, port));
};
await client.LoginUserIfNeeded();

Use the existing WTelegram.Client and import the chosen compatible SOCKS library. proxyHost, port and account identify the intermediary; the lambda's address/port are WTelegram's requested destinations.

CreateConnection must return a connected TcpClient after proxy negotiation. Reaching only the proxy port does not provide that handoff.

The example uses a synchronous library method. Wrapping its result in Task.FromResult does not make the connection nonblocking; use a supported asynchronous method when needed.

Retain the handler for later connections as well as login. Telegram account authentication remains separate from the SOCKS account.

Erthyglow kevrennek