Program.cs

using Microsoft.Extensions.Configuration;
using SimpleInjector;
using SimpleInjector.Lifestyles;
using SimpleInjectorExample;
using SlackNet.Events;
using SlackNet.SimpleInjector;

Console.WriteLine("Configuring...");

var settings = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json")
    .AddUserSecrets<Program>()
    .Build()
    .Get<AppSettings>();

await using var container = new Container { Options = { DefaultScopedLifestyle = new AsyncScopedLifestyle() } };

container.AddSlackNet(c => c
    // Configure the tokens used to authenticate with Slack
    .UseApiToken(settings.ApiToken) // This gets used by the API client
    .UseAppLevelToken(settings.AppLevelToken) // This gets used by the socket mode client

    // Register your Slack handlers here
    .RegisterEventHandler<MessageEvent, PingDemo>()
);

Console.WriteLine("Connecting...");

var client = container.SlackServices().GetSocketModeClient(); // .SlackServices() is a convenience method for resolving SlackNet services, but you can also resolve ISlackSocketModeClient directly
await client.Connect();

Console.WriteLine("Connected. Press any key to exit...");
await Task.Run(Console.ReadKey);

record AppSettings
{
    public string ApiToken { get; init; } = string.Empty;
    public string AppLevelToken { get; init; } = string.Empty;
}