Add meta tag to search results

Well I am asuming you want to amend the <title> tag?

Add this in functions.php

  add_filter('wp_title', 'my_custom_page_title');

  function my_custom_page_title($title) { 
    global $s;
    if( isset($_REQUEST['author_name']) && is_search() ) {
            $title="Search Results for ".$s.' from author '.$_REQUEST['author_name'];
    }
    return $title;  
  } 

If $s returns blank replace that with $_REQUEST['s']