If you need the linkbutton's OnClick server-side event to fire, you need to use __doPostback(eventTarget, eventArgument). Here is the sample code:
<asp:LinkButton ID="btnMyButton" runat="Server" OnClick="Button_Click" />
<script type="text/javascript">
function onMyClientClick()
{
//'click' the link button, form will post, Button_Click will fire on back-end
//the second parameter is required and superfluous, just use blank
__doPostBack('<%=btnMyButton.UniqueID%>', '');
}
</script>