Set textbox value while we are on another form

public partial class Form1 : Form
   {
      public Form1()
      {
         InitializeComponent();
      }

      private void button1_Click(object sender, EventArgs e)
      {
         Form2 ChildFrm = new Form2(this);
         ChildFrm.ShowDialog();
      }
   }

public partial class Form2 : Form
   {
      private Form1 ParentForm;
      public Form2(Form1 parentForm)
      {
         InitializeComponent();
         ParentForm = parentForm;
      }

      private void Form2_FormClosing(object sender, FormClosingEventArgs e)
      {
         ParentForm.Controls["ParentFormTextBox"].Text = ChildTextBox.Text;
      }
   }

namespace form1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click_1(object sender, EventArgs e)
        {
            Form2 frm = new Form2(textBox1);
            frm.Show();
        }
    }
}

namespace form1
{
    public partial class Form2 : Form
    {
        TextBox txtForm1;
        public Form2()
        {
            InitializeComponent();
        }
        public Form2(TextBox txt)
        {
            InitializeComponent();
            txtForm1 = txt;
        }
        private void Form2_Load(object sender, EventArgs e)
        {
            txtForm1.Text = "hello world";
        }
    }
}