Assign a Textbox in the parent page from a ModalPopup in asp.net

I see a lot of people have been asking this question, some was because of the if postback check , but mine is different. i am using AjaxModalExtender to show my popup on the server side.
the breakpoints got a hit and i stepped through it and i the value get assigned to the textbox <b> txtreference.Text</b> , but when i close the modal the textbox is empty , or while the modal is open i can see the value does not reflect in the parent page. Here is the Code for invoking the modal


[code]protected void Button1_Click(object sender, EventArgs e)
{
AjaxControlToolkit.ModalPopupExtender modalPop = new AjaxControlToolkit.ModalPopupExtender();

modalPop.ID = "popUp";

modalPop.PopupControlID = "ModalPanel";

modalPop.TargetControlID = "Button1";

modalPop.DropShadow = true;

modalPop.BackgroundCssClass = "modalBackground";

modalPop.CancelControlID = "btnCancel";

this.Panel1.Controls.Add(modalPop);

modalPop.Show();
}
[/code]

Thanks

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