How can a subform inside a panel of parentform access parent form controls on button click

I have a parent form which has a textbox1 and a panel1 and a button1. I open another form(say form2) in panel1 on button1 click. form2 has a textbox and button. when i enter a value in textbox and click on the button in subform , the textboxvalue of subform should be copied to textbox value of parent form and panel1 should become invisible.

I use the following code, For button1 click(of parentform),

        panel1.Visible = true;
        Form2 f2 = new Form2();
        f2.TopLevel = false;
        f2.AutoScroll = true;
        panel1.Location = new Point(this.ClientSize.Width / 2 - panel1.Size.Width / 2, this.ClientSize.Height / 2 - panel1.Size.Height / 2);
        panel1.Anchor = AnchorStyles.None;
        panel1.Controls.Add(sp);
        f2.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;
        f2.Dock = DockStyle.Fill;
        f2.usrnam = this.usrnam;
        f2.connectionstring = this.connectionstring;
        f2.Show();

For button click of subform,

        string s = textBox1.Text;
        Form1 f1= new Form1(); /* However this line is wrong , I donot want to initialize the form again i just need a way to access Form1 controls */
        f1.panel1.Visible = false;
        f1.textBox1.Text = s;