Google Tag Manager
As a custom HTML tag. The last resort.
Create a custom HTML tag
Tags → New → Tag configuration → Custom HTML.
Paste the key
As is, with the defer attribute and your domain.
html<script defer data-domain="example.com" src="https://pisi.to/js/llave.js"></script>Trigger: All Pages
All Pages is enough. Do not add a history-change trigger: the key already handles SPA navigations and you would get duplicate pageviews.
Submit the container
GTM changes do nothing until you hit Submit and publish the version.
Check that it works
Open your site in a new tab and watch the dashboard: you should show up under "home right now" in under ten seconds. If you do not, run through the onboarding checklist.