Pre populate Custom Menus

add-menu.php

<?php

add_action( 'init', 'add_menus' ); 

function add_menus() { 	
	
	register_nav_menus( array( 'main_nav' =--> 'The main menu', ) );
	
	$new_menu_id = wp_create_nav_menu( 'Main Menu' );
	
	$page_args = array(
		'menu-item-object-id' 	=> 1,
		'menu-item-object' 		=> 'page',
		'menu-item-parent-id' 	=> 0,
		'menu-item-type' 		=> 'post_type',
		'menu-item-title' 		=> 'THIS WILL BE THE LINK TITLE',
		'menu-item-status' 		=> 'publish',
	);
	if ( $new_menu_id > 0 ) {
		
		// set our new MENU up at our theme's nav menu location
		set_theme_mod( 'nav_menu_locations' , array( 'main_nav' => $new_menu_id ) );
		
		// add a menu item to that new menu
		wp_update_nav_menu_item( $new_menu_id , 0, $page_args );
	}
}