If you are writing a Windows Forms App, one way to do it is to create a property for the textbox on your Form2, like this.
public string TextBoxValue
{
get
{
return TextBox1.Text;
}
set
{
TextBox1.Text = value;
}
}
And then, you can set values on your Form1, like this.
Form2 f2 = new Form2();
f2.TextBoxValue = "YourValue";
f2.Show();