Add post type menu as admin_sub_menu

Post type menu in admin submenu

// 1. when registering a custom post type set show_in_menu to false,
function ttlm_register_team_custom_type() {
	$labels = array(
		'name'               => _x( 'Teams', 'post type general name', 'your-plugin-textdomain' ),
		'singular_name'      => _x( 'Team', 'post type singular name', 'your-plugin-textdomain' ),
		'menu_name'          => _x( 'Teams', 'admin menu', 'your-plugin-textdomain' ),
		'name_admin_bar'     => _x( 'Team', 'add new on admin bar', 'your-plugin-textdomain' ),
		'add_new'            => _x( 'Add New', 'team', 'your-plugin-textdomain' ),
		'add_new_item'       => __( 'Add New Team', 'your-plugin-textdomain' ),
		'new_item'           => __( 'New Team', 'your-plugin-textdomain' ),
		'edit_item'          => __( 'Edit Team', 'your-plugin-textdomain' ),
		'view_item'          => __( 'View Team', 'your-plugin-textdomain' ),
		'all_items'          => __( 'All Teams', 'your-plugin-textdomain' ),
		'search_items'       => __( 'Search Teams', 'your-plugin-textdomain' ),
		'parent_item_colon'  => __( 'Parent Teams:', 'your-plugin-textdomain' ),
		'not_found'          => __( 'No teams found.', 'your-plugin-textdomain' ),
		'not_found_in_trash' => __( 'No teams found in Trash.', 'your-plugin-textdomain' )
	);

	$args = array(
		'labels'             => $labels,
		'public'             => true,
		'publicly_queryable' => true,
		'show_ui'            => true,
		'show_in_menu'       => false, //<--- HERE
		'query_var'          => true,
		'rewrite'            => array( 'slug' => 'ttlm_team' ),
		'capability_type'    => 'post',
		'has_archive'        => true,
		'hierarchical'       => false,
		'menu_position'      => null,
		'supports'           => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' )
	);

	register_post_type( 'ttlm_team', $args );
}
add_action( 'init', 'ttlm_register_team_custom_type' );


// 2. when adding a sub-menu page set the callback function to NULL and
// set the menu_slug to the appropriate custom post type url 
// (in this case it's edit.php?post_type=ttlm_team)
add_submenu_page( 'ttlm', 'TTLM Teams', 'Teams', 
'manage_options', 'edit.php?post_type=ttlm_team', NULL );


when registering a custom post type set the ‘show_in_menu’ argument to false,
when adding a sub-menu item set the callback function to NULL and the menu_slug to the custom post type, 
e.g. ‘edit.php?post_type=your_custom_post_type_name_goes_here’