How to add custom link as menu Item programmatically in WordPress Menu?

function.php

<?php 	
/**
 * To add a custom link as custom menu item in WordPress menu
 */
 $menu_id = 4; // WordPress menu ID.
wp_update_nav_menu_item($menu_id, 0, array(
        'menu-item-title' =>  __('Home'),
        'menu-item-classes' => 'home',
        'menu-item-url' => 'http://buddydevelopers.com', 
        'menu-item-status' => 'publish'
        'menu-item-type' => 'custom',
//         'menu-item-target' => '_blank',
//         'menu-item-xfn' => 'friend'
    )
);

To read more please visit link below.
http://buddydevelopers.com/how-to-add-menu-item-programmatically-in-wordpress/