[STAThreadAttribute]
 static void Main()
{Application.EnableVisualStyles();              
  Application.SetCompatibleTextRenderingDefault(false);              
  Application.Run(new Form_Main());              
}
//кнопка по которой открывается форма
private void mMain_Setting_Click(object sender, EventArgs e)
        {Form_Setting frmSetting = new Form_Setting();                        
         frmSetting.Owner=this; 
         frmSetting.ShowDialog();}

 public partial class Form_Setting : Form
    {              
        public Form_Setting()
        {
            InitializeComponent();
        }
      private void pb_dir_out_Click(object sender, EventArgs e)
        {folderBrowserDialog1.SelectedPath = Directory.GetCurrentDirectory();
            if (folderBrowserDialog1.ShowDialog() == DialogResult.OK)
            {   
                this.tb_set_dir_out.Text = folderBrowserDialog1.SelectedPath;
            }
         }
 }

private void show()
        { folderBrowserDialog1.ShowDialog()
        }
private void pb_dir_out_Click(object sender, EventArgs e)
{Thread tmpThread1 = new Thread(new ThreadStart(show));
 tmpThread1.SetApartmentState(ApartmentState.STA);
 tmpThread1.Start();
}

private string Show_Dialog()
        {
            string v_selectedPath="";
            var t = new Thread((ThreadStart)(() =>
            {
                FolderBrowserDialog fbd = new FolderBrowserDialog();
                fbd.SelectedPath = Directory.GetCurrentDirectory();
                fbd.ShowNewFolderButton = true;
                if (fbd.ShowDialog() == DialogResult.Cancel)
                return;
                v_selectedPath = fbd.SelectedPath;
            }));
            t.SetApartmentState(ApartmentState.STA);
            t.Start();
            t.Join();
            return v_selectedPath;
        }

[STAThread]
static void InitParametrPO()
{//бла бла
}

 static void Main()
 {Application.EnableVisualStyles();              
   Application.SetCompatibleTextRenderingDefault(false);              
   InitParametrPO();              
   Application.Run(new Form_Main());
}