Make an on user visit pop-up

I want to make a pop-up window that immediately appears in the middle of the site on user visit https://demo.infyways.com/demo/popup-and-iframes/website-disclaimer-popup

This is an example site of what I want but change the pop-up background. I know that this site is a pop-up generator but whenever I copy the code in generates I get my msg on the top of my site I only want the pop-up when user visits

This is a very mundane example of what I am trying to achieve using query

<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
   <script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js">.  </script>
    <script>
    $( function() {
   $( "#dialog" ).dialog();
  } );
 </script>

<div id="dialog" title="Basic dialog">
 <p>This is the default dialog which is useful for displaying     information. The dialog window can 
 be moved, resized and closed with the      'x' icon.</p>
 </div>

I can't seem to remove the close option from it and make it so the user can't proceed if he doesn't accept. I want to create 2 buttons 1 for agreeing -> user proceeds to my site which isn't visible if he doesn't agree 2nd button disagree and there is just a blank page Can i use query or am I wasting my time for the agree or disagree buttons