How to add custom menu item ?

It is available to add anything you want, from tiny image to custom HTML elements in menu. You should define for which menu to add custom content. In this example, custom text is added to ‘main_menu’.

add_filter( 'wp_nav_menu_items', 'your_custom_menu_item', 10, 2 );
function your_custom_menu_item ( $items, $args ) 
{
    if ($args->theme_location == 'main_menu') 
        {

            $items .= 'Show whatever'; 
            return $items;
        }
}