Add WordPress custom post type as submenu item of existing admin menu.

wp-custom-post-type-as-submenu-item.php

<?php
if (! function_exists('gist_register_cpt')) {

    /** 
     * Register Custom Post Type
     */
    function gist_register_cpt() {

        // Set CPT labels
        $labels = array(
            'name'                  => _x('Items', 'Post Type General Name', 'textdomain'),
            'singular_name'         => _x('Item', 'Post Type Singular Name', 'textdomain'),
            'menu_name'             => __('Items', 'textdomain'),
            'name_admin_bar'        => __('Item', 'textdomain'),
            'archives'              => __('Item Archives', 'textdomain'),
            'parent_item_colon'     => __('Parent Item' . ':', 'textdomain'),
            'all_items'             => __('All Items', 'textdomain'),
            'add_new_item'          => __('Add New Item', 'textdomain'),
            'add_new'               => __('Add New', 'textdomain'),
            'new_item'              => __('New Item', 'textdomain'),
            'edit_item'             => __('Edit Item', 'textdomain'),
            'update_item'           => __('Update Item', 'textdomain'),
            'view_item'             => __('View Item', 'textdomain'),
            'search_items'          => __('Search Items', 'textdomain'),
            'not_found'             => __('Not found', 'textdomain'),
            'not_found_in_trash'    => __('Not found in Trash', 'textdomain'),
            'featured_image'        => __('Featured Image', 'textdomain'),
            'set_featured_image'    => __('Set featured image', 'textdomain'),
            'remove_featured_image' => __('Remove featured image', 'textdomain'),
            'use_featured_image'    => __('Use as featured image', 'textdomain'),
            'insert_into_item'      => __('Insert into Item', 'textdomain'),
            'uploaded_to_this_item' => __('Uploaded to this Item', 'textdomain'),
            'items_list'            => __('Items list', 'textdomain'),
            'items_list_navigation' => __('Items list navigation', 'textdomain'),
            'filter_items_list'     => __('Filter Items list', 'textdomain'),
        );

        // Set CPT args
        $args = array(
            'label'                 => __('Item', 'textdomain'),
            'description'           => __('Custom Items Collection', 'textdomain'),
            'labels'                => $labels,
            'supports'              => array('title', 'editor', 'excerpt', 'author', 'thumbnail', 'comments', 'trackbacks', 'revisions', 'custom-fields', 'page-attributes', 'post-formats'),
            'hierarchical'          => false,
            'public'                => true,
            'show_ui'               => true,
            'show_in_menu'          => false, // Set `show_in_menu` to false to prevent WP from adding a new parent menu item.
            'menu_position'         => 5,
            'menu_icon'             => 'dashicons-editor-code',
            'show_in_admin_bar'     => false,
            'show_in_nav_menus'     => false,
            'can_export'            => true,
            'has_archive'           => false,		
            'exclude_from_search'   => true,
            'publicly_queryable'    => true,
            'rewrite'               => false,
            'capability_type'       => 'page',
        );

        // Register the CPT
        register_post_type('gist_post_type_key', $args);

    }

    add_action('init', 'gist_register_cpt', 0);

    /**
     * Add new CPT to an existing menu with add_submenu_page
     *
     * Docs: https://developer.wordpress.org/reference/functions/add_submenu_page/
     */
    function gist_add_cpt_to_submenu() {

        /**
         * $parent_item is the slug of the parent menu item you want to place your CPT under.
         *
         * Possibilities:
         * - Dashboard: ‘index.php’
         * - Posts: ‘edit.php’
         * - Media: ‘upload.php’
         * - Pages: ‘edit.php?post_type=page’
         * - Comments: ‘edit-comments.php’
         * - Custom Post Types: ‘edit.php?post_type=your_post_type’
         * - Appearance: ‘themes.php’
         * - Plugins: ‘plugins.php’
         * - Users: ‘users.php’
         * - Tools: ‘tools.php’
         * - Settings: ‘options-general.php’
         * - Network Settings: ‘settings.php’
         * - Admin Page: ’your-admin-page-slug’ (ex: admin.php?page=your-admin-page-slug)
         */

        $parent_item = 'parent-slug';

        add_submenu_page($parent_item, 'Items Page Title', 'Items Menu Name', 'edit_pages', 'edit.php?post_type=gist_post_type_key'); 
    
    }

    add_action('admin_menu', 'gist_add_cpt_to_submenu', 10); // If you have multiple submenu items, modify the priority to move it around.

}