Squarespace

Paste the snippet once, in Settings, Advanced, Code Injection, in the Footer field, then publish the site.

<script async src="https://js.flaazecdn.com/v1/loader.js" data-key="pk_live_REPLACE_WITH_YOUR_PUBLISHABLE_KEY"></script>

The loader renders nothing until the tenant is active (or sandbox with a pk_test_ key), so it is safe to publish before go-live.