Inside a modalpopupextender panel, you need to add and set the UseSubmitBehavior to false for the OK/Cancel controls to be enabled with postback to behave outside of the ajax control.
Example:
<asp:LinkButton ID="OkButton" runat="server" Text="Button Text" UseSubmitBehavior="false" PostBackUrl="product.aspx?id=3" />
You can also use OnClick in the button to perform an action outside of ajax with UseSubmitBehavior=”false”.
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Title talks about an asp:linkbutton but the code uses an asp:button. ?????
Yes. Thank you for the catch. There was a typo which is now fixed. Hope this post helped you in your coding.