WordPress add attachment to auto-save

autosave-attachment.php

<?php
  
add_action('save_post', 'add_attachment_to_autosave');

function add_attachment_to_autosave( $post )
{
  if( !wp_is_post_autosave($post) ) :
  	return;
  endif;
  
  if ( get_post_meta($post->ID, '_thumbnail_id', true) ) :
  	return;
  endif;
  
  $attachment_id = 999;
  
  add_post_meta($post->ID, '_thumbnail_id', $attachment_id);
  
}