Pass Data from modalpopupextender window's gridview to parent textbox

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C#

ASP.NET

ModalPopUpExtender

Pretty sure people would have come across this.

Issue:
There is a search customer button, on clicking it, opens a modal window based on modalpopupextender.
On the modal window, search results are loaded into a gridview and the user is allowed to select the desired customer.

Now how do I pass those selected customer details to textboxes in parent window.
Right now I tried passing the value in the rowselected event of the gridview, but Iam not able to access the parent form's textbox in that way.

Would be great if someone can direct me in the right direction.