Adding Custom Script Code to HTML Header of site pages

Hello, I have taken over management of a Treepl website, and need help in finding where to add custom code to website HTML header of site pages. I need to add a custom tracking code for a video ad company, like so:
*< script type=“text/javascript” async src=“https://websitename.com/gm.js?id=111” >< /script > (example)

Where would I be able to add this so that appears on every page for accurate tracking? I’m sure I’m missing it since this is a standard feature for website CMSs, but I’m lost.

I appreciate the help. Thank you!

Page templates are under the Content > Content templates. Depending on how the site was built there may be multiple templates. So you may need to ad it to more than one. The head elements could also be added to the templates with a snippets or an include.