FrmMain frmBM = new Bookmark_Control.FrmMain();
frmBM.ShowDialog(this);

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;
namespace FormTest
{
    static class Program
    {
        /// <summary>
        /// Der Haupteinstiegspunkt für die Anwendung.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new frmBookmark());
        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace FormTest
{
    public partial class frmBookmark : Form
    {
        public frmBookmark()
        {
            InitializeComponent();
        }
 
        private void frmBookmark_Load(object sender, EventArgs e)
        {
            Bookmark_Control.FrmMain frmBM = new Bookmark_Control.FrmMain();
 
            frmBM.ShowDialog(this);
 
        }
    }
}