Add a custom menu

fucntions.php

<?php
// Custom Menu
add_filter( 'init', 'register_primary_menu');
function register_primary_menu() {
    register_nav_menu('nav-primary',__( 'Navigation Primary' ));
}
function custom_primary_menu() {
    $menu_name = 'nav-primary';

    if (($locations = get_nav_menu_locations()) && isset($locations[$menu_name])) {
	$menu = wp_get_nav_menu_object($locations[$menu_name]);
	$menu_items = wp_get_nav_menu_items($menu->term_id);

	$menu_list = '<ul>' ."\n";
	foreach ((array) $menu_items as $key => $menu_item) {
	    $title = $menu_item->title;
	    $url = $menu_item->url;
	    $menu_list .= '<li><a ';
	    $menu_list .= ' href="'. $url .'">'. $title .'</a>  <i class="fa fa-plus-square" aria-hidden="true"></i></li>' ."\n";	    
	}
	$menu_list .= '<li><form method="get" action="/search/"><input type="text" name="q" placeholder="Search Next Level.."/></form></li>';
	$menu_list .= '</ul>' ."\n";
    } else {
	$menu_list = 'Please set up your menu';
    }
    echo $menu_list;
}