How to add page/post as menu Item programmatically in WordPress?

function.php

<?php 	
/**
 * To add a page/post in WordPress menu
 */
$menu_id = 4; //menu ID.
$_post_id = 5; // post/page ID.
// note here $menu_item_db_id is already added menu item ID, if it will be 0 new menu will be added otherwise we can edit previous menu with code.
$menu_response_data = wp_update_nav_menu_item( $menu_id , $menu_item_db_id=0, array(
		'menu-item-object-id' => $_post_id,
		'menu-item-object' => 'page',
		'menu-item-title' =>  wp_strip_all_tags( __( 'My Page', 'awesome-support' ) ),
		'menu-item-status' => 'publish',
		'menu-item-type' => 'post_type'
	)
);

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