How to add custom post type under option page

You can add custom post type in Settings menu by passing ‘options-general.php’ value to ‘show_in_menu’ parameter as shown below

Example : 'show_in_menu' => 'options-general.php'

Full Code :

function codex_custom_init() {
  $labels = array(
'name' => 'Books',
'singular_name' => 'Book',
'add_new' => 'Add New',
'add_new_item' => 'Add New Book',
'edit_item' => 'Edit Book',
'new_item' => 'New Book',
'all_items' => 'All Books',
'view_item' => 'View Book',
'search_items' => 'Search Books',
'not_found' =>  'No books found',
'not_found_in_trash' => 'No books found in Trash', 
'parent_item_colon' => '',
'menu_name' => 'Books'
  );

  $args = array(
'labels' => $labels,
'public' => true,
'publicly_queryable' => true,
'show_ui' => true, 
'show_in_menu' => 'options-general.php',
'query_var' => true,
'rewrite' => array( 'slug' => 'book' ),
'capability_type' => 'post',
'has_archive' => true, 
'hierarchical' => false,
'menu_position' => null,
'supports' => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' )
  ); 

  register_post_type( 'book', $args );
}
add_action( 'init', 'codex_custom_init' );

For more information refer this page.