[Plugin: Related Posts Thumbnails] How to view related posts in old posts?

Hi,
I love this plugin, but I’d like to avoid to write this code (<?php get_related_posts_thumbnails(); ?>) in each post…there’s a way to do it automatically and extend the plugin funcionality to the old posts?
Thanks a lot
Roberto

https://www.ads-software.com/extend/plugins/related-posts-thumbnails/