MessageBox in ASP.NET does not exist. You have to use
JavaScript
to render a popup in the browser or use client-side library like jQuery, bootstrap etc... Fortunately, WebForms offers
AjaxControlToolkit controls which is a wrapper for standard ASP.NET Server Controls. The AjaxControlToolkit control has a
ModalPopupExtender [
^]that you can use to display a modal without writing JavaScript yourself.
If you want a pre-built MessageBox like controls in ASP.NET, then you might want to try out my opensource control that I've built many years ago here:
A Customized MessageBox, ConfirmBox and FrameBox Controls For ASP.NET WebForms[
^]