you can pass as parameter the textbox of the Form1, like this:
On Form 1 buttom handler:
private void button2_Click(object sender, EventArgs e)
{
Form2 newWindow = new Form2(textBoxForReturnValue);
newWindow.Show();
}
On the Form 2
public static TextBox textBox2; // class atribute
public Form2(TextBox textBoxForReturnValue)
{
textBox2= textBoxForReturnValue;
}
private void btnClose_Click(object sender, EventArgs e)
{
textBox2.Text = dataGridView1.CurrentCell.Value.ToString().Trim();
this.Close();
}