Add search form to the menu item

fun.php

add_filter( 'wp_nav_menu_items','dt_corpo_search_box_to_menu', 10, 2 );

function dt_corpo_search_box_to_menu( $items, $args ) {
  if( $args->theme_location == 'main' )
    $items .= '<li id="menu-search-form">' . get_search_form( false ) . '</li>';
  return $items;
}