C#

   void UIThread()
   {
      Form yourForm = new Form();
      Application.Run(yourForm);
   }
}

class SecondForm
{
   private TextBox tbText = new TextBox();
   
   public string TbText
   {
       get { return tbText.Text; }
       set { tbText.Text = value; }
   }
}

class MainForm
{
  SecondForm secondForm;

  public void ShowSecondForm()
  {
     secondForm = new SecondForm();
     secondForm.Show(); // or ShowDialog
  }

  public void SetText(string text)
  {
     secondForm.TbText = text;
  }
}