Application.Run(new MyForm())

var form = new MyForm();

form.Show(); // или form.ShowDialog();

static void Main(string[] args)
        {
            AppDomain applicationDomain = System.AppDomain.CreateDomain("IDGAppDomain");
            applicationDomain.ExecuteAssembly(@"D:\Projects\TestCode.exe");
 Console.WriteLine("Press any key to unload the application domain...");
            Console.ReadKey();
            System.AppDomain.Unload(applicationDomain);
        }
    }
}

using System;
using System.Windows.Forms;
using Megatec.Framework.Plugins;

namespace MyApplication
{
    [Plugin("Мой плагин", Forms = new string[] { "frmGroup", "frmManager" })]
    public sealed class Plugin
    {
        [PluginMethod("Мой плагин")]
        public void Run()
        {
            var connectionString = Megatec.MasterTour.DataAccess.Manager.ConnectionString;
            var form = new MainForm(connectionString);

            if (!form.IsDisposed)
                form.Show();
        }
    }
}