Can i add custom code in Source in specific page, Header from function.php?

You can add conditions in you function if you want to add icons like this method you mentioned in question.

function hook_link() {

  global $post;
  if ($post->post_type == 'page' && $post->ID == 'whatever page id') {
    echo '<link href="http://buhehe.de/wp-content/uploads/2017/12/Buhehe.ico" rel="shortcut icon" type="image/x-icon">';
  }
  ?>

  <?php
}

add_action('wp_head', 'hook_link');

to check page or post etc you can use is_page() and is_single() functions.