Parent/Child form issue

Question

Hello,

i have the following code:

   private void searchDataGridView_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
            string passIDNO;
            passIDNO = searchDataGridView.Rows[e.RowIndex].Cells[1].Value.ToString();
            Form1 form1 = new Form1(passIDNO, 1);
            form1.button1_Click(sender, e); // Retrieves data of Customer to Form1
            //form1.MDIParent = ????
            form1.Show();
         }

I have a ParentForm called MainForm and all my forms open inside MainForm. How can i make Form1 (which is called from Form2) opens inside MainForm as well?
(Form2 opens inside MainForm)

Thank you in advance.