This is how you use Reflection to change TwitchLib-servers.

LessHacky.cs

public class FDGTClient : WebSocketClient
{
	public FDGTClient() :base()
	{
		var bindingFlags = BindingFlags.NonPublic | BindingFlags.Instance;
		var urlField = base.GetType().BaseType.GetFields(bindingFlags).FirstOrDefault(f => f.Name.Contains("Url"));
		urlField.SetValue(this, "wss://irc.fdgt.dev:443");
	}
}

void Main()
{
	// Initialize the client as normal, but without any oauth
	TwitchClient client = new TwitchClient(new FDGTClient());
	client.Initialize(new ConnectionCredentials("fdgttest", "", "wss://irc.fdgt.dev:443", disableUsernameCheck: true), "fdgt");

	var channel = string.Empty;

	client.OnJoinedChannel += (sender, args) =>
	{
		channel = args.Channel;
	};
	
	client.OnLog += (sender, args) => {
		Console.WriteLine(args.Data);	
	};

	client.Connect();
	
	var lastLine = string.Empty;
	
	while(lastLine != "exit")
	{
		lastLine = Console.ReadLine();
		client.SendMessage(channel, lastLine);
	}
	
	client.Disconnect();
}

TwitchLib-fdgt.cs

/* Installed Nuget-packages are only TwitchLib */

using TwitchLib.Client;
using TwitchLib.Communication.Clients;
using TwitchLib.Communication.Models;
using TwitchLib.Client.Models;
using TwitchLib.Client.Enums;
using TwitchLib.Communication.Interfaces;

void Main()
{
	// Initialize the client as normal, but without any oauth
	TwitchClient client = new TwitchClient(protocol: ClientProtocol.WebSocket);
	client.Initialize(new ConnectionCredentials("fdgttest", "", disableUsernameCheck: true), "fdgt");
	
	var bindingFlags = BindingFlags.NonPublic | BindingFlags.Instance;
	// Fetch the private field _client from the TwitchClient instance and then grab the field that contains "Url", since it doesn't have a public setter
	IClient _client = (IClient)client.GetType().GetField("_client", bindingFlags).GetValue(client);
	var serverField = _client.GetType().GetFields(bindingFlags).FirstOrDefault(f => f.Name.Contains("Url"));
	// Set the new value
	serverField.SetValue(_client, "wss://irc.fdgt.dev:443");
	
	var channel = string.Empty;

	client.OnJoinedChannel += (sender, args) =>
	{
		channel = args.Channel;
	};
	
	client.OnLog += (sender, args) => {
		Console.WriteLine(args.Data);	
	};

	client.Connect();
	
	var lastLine = string.Empty;
	
	// Lets us input commands manually for testing of events, until we enter "exit"
	while(lastLine != "exit")
	{
		lastLine = Console.ReadLine();
		client.SendMessage(channel, lastLine);
	}
	
	client.Disconnect();
}