title: Place a custom post type under a custom menu items
body: <p>I'm trying to place a custom post type under the menus I made, Here is the screenshot of the menus. I want to add the custom post type together with that Menus. I used sandbox_theme_menu, but the custom post type is not appearing, <br><br></p>
  
  <p>I tried adding this line in my register_post_type but still not appearing. Hmm any idea?</p>
  
  <pre><code>'show_in_menu' =&gt; 'edit.php?page=sandbox_theme_menu',
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/AKgKd.png" alt="enter image description here"></p>