You've got the function to pass the text from form1 to form2 but you also need something to pass the amended information from form2 back to form1 e.g. in form1
public void SetNewListBoxValue(string newValue)
{
listBox1.Items.Add(newValue);
}
and in form2
private void okButton_Click(object sender, EventArgs e)
{
f1.SetNewListBoxValue(textBox1.Text);
}