Squarespace has no plugin. Code Injection in the header is the whole install.
1. Copy the snippet
In Stellar, open the project for this site and copy the snippet from your domain settings. Leave Anti-Flicker on if you are running visual tests. Direct head install is what makes that work. How anti-flicker works.
2. Paste it on Squarespace
Open Website, then Pages.
Scroll to the bottom of that panel, to Custom Code, and open Code Injection. On older sites the same panel is under Settings, then Advanced, then Code Injection.
Paste the snippet into Header. Leave Footer empty.
Save.
Open a published page in a normal browser window. The Squarespace editor often hides injected scripts.
Code Injection needs a Core, Plus, or Advanced plan, or an older Business or Commerce plan. Basic and Personal plans cannot add scripts to the header. A Code block on one page is the wrong place. It only covers that page and loads too late.
3. Confirm it loaded
Open a published page in a private browser window. Editor and draft previews often hide the snippet.
Open developer tools and go to the Network tab.
Reload and look for stellar.js. If that request is there, the install is done.
If it does not load
If Code Injection is missing, check the plan. Basic and Personal do not include it.
Paste into Header, not Footer, and not a Code block.
If you manage more than one Squarespace site, confirm you saved it on the right one.
Give it a minute, then hard-refresh a published page. The editor preview is not a valid check.