à In Page_Load event of form2.aspx.cs, add following code :

Button1.Attributes.Add("onClick", "return textTransfer();");

à In head tag of form2.aspx add following code :

<script language="javascript">

    function textTransfer() {

    debugger;

    var doc=document.forms[0];

    var txt=Label1.innerHTML;

    window.open ("form1.aspx?val="+txt,"mywindow","scrollbars=no,resizable=yes,width=400,height=280");

    }

    </script>

à In Page_Load event of form1.aspx.cs, add following code :

TextBox1.Text = Request.QueryString["val"].ToString();