Adding a JS Code Snippet

I'm trying to install a social proof site widget and the instructions are the following:

Copy and paste the following JS Code Snippet before the end of the </head> of your website.

<!-- Pixel Code-->
<script async src="XXXXXXXXX"></script>
<!-- END Pixel Code →

What's the best way to add this code snippet? Would it be to create a new Hook in Elements, then select wp_head? Do I check Execute Shortcodes or PHP? How do I determine what priority to use?

Thanks!