Show Admin Menu

add_submenu_page() this function used to add submenu in Admin Menu Page. And You will remove submenu from wordpress admin using remove_submenu_page().

add_submenu_page() by default takes 7 parameters.

add_submenu_page( 
      string $parent_slug, 
      string $page_title, 
      string $menu_title, 
      string $capability, 
      string $menu_slug, 
      callable $function = '',
      int $position = null 
)

Write the following code in your functions.php file to create custom post type.

//admin menu callback function

function coupon_register_submenu_page() {

    //Add Offer Configuration Sub Menu   
    add_submenu_page('edit.php?post_type=coupon', 'coupon configuration', 'Coupon Configuration', "manage_options", 'coupon_configuration', 'couponconfiguration', '');

    //Add Deal Configuration Sub Menu
    add_submenu_page('edit.php?post_type=coupon', 'deal configuration', 'Deal Configuration', "manage_options", 'deal_configuration', 'dealconfiguration', '');

    //Add Custom Social Sharing Sub Menu
    add_submenu_page('edit.php?post_type=coupon', 'deal configuration', 'Social Options', "manage_options", 'social-share', 'social_share_page', '');
    
}
add_action('admin_menu', 'coupon_register_submenu_page');

//add submenu page callback function

function couponconfiguration(){
    echo 'Offer Configuration';
    return;
}

function dealconfiguration(){
    echo 'DEAL Configuration';
    return;
}

function social_share_page(){
    echo 'Social Sharing Configuration';
    return;
}