Show and Hide ModalPopupExtender from JavaScript
1) Assign a BehaviourID to the ModalPopupExtender using the BehaviourID attribute.
BehaviorID ="ModalBehaviour"
2) Use the $find method to get a handle to the Modal Popup Behaviour .
$find ("ModalBehaviour").
3) Call your hide and show methods on the acquired handle.
4) The Javascript would look like this.
<script language="javascript"> function ShowModalPopup() { $find("ModalBehaviour").show(); } function HideModalPopup() { $find("ModalBehaviour").hide(); } </script>