How to load scripts on custom non-header page?

We have a custom wordpress page that does not load the regular wordpress header.
What do I have to paste in this custom page’s header to make sure lazy load works?

I tried:

<script type=’text/javascript’ src=’https://www.domain.com/wp-content/plugins/bj-lazy-load/js/combined.min.js?ver=0.6.10′></script&gt;

But it does not work to echo an image:

<?php the_post_thumbnail(); ?>

Please help.

Thanks.

https://www.ads-software.com/plugins/bj-lazy-load/