Call server postback from ModalPopup extender

The Atlas AJAX Toolkit has a very convenient ModalPopup extender. However by default it doesn't do post back on button clicks, even the event is wired with the button:

< runat="server" id="btnAddExistingPart" text="Add" cssclass="SplashButton" onclick="btnAddExistingPart_Click">

Upon click it only dismiss the modal popup.

You have to explicitly call the post back client method to initiate the post back:

protected void Page_Load(object sender, EventArgs e)
{
...
btnAddExistingPart.OnClientClick = "__doPostBack('" + btnAddExistingPart.UniqueID + "','')";
...
}