How to insert meta keyword to search result page

that is relatively easy. you can add this to your theme’s functions.php file

add_action('wp_head', 'add_meta_tags');
function add_meta_tags(){

  if(is_search()){
    $search_keyword = get_search_query();
    echo '<meta name="META_NAME" content="META TESTING" />';
  }
}

you may change echo '<meta name="META_NAME" content="META TESTING" />'; as you want to output the meta tags.

however, your theme needs to call wp_head(); for this to work. check your theme’s header.php file.