Squarespace calls this code injection, and the site-wide header box is exactly what you want. It is one screen and it applies to every page.
Code injection
Open Settings, then Advanced, then Code Injection. On newer sites this sits under the website settings rather than the account settings.
Paste the structured data into the Header box. Include the
<script>tags. The Header box is injected into<head>on every page of the site.Save. Code injection applies to the live site immediately rather than on next publish.
There is a separate per-page code injection under each page’s settings. Use the site-wide one for structured data describing the business, which is true on every page rather than one. The per-page box is for share preview tags, below.
Code injection is part of the Core, Plus and Advanced plans. If you do not see it, that is why, and the share settings below still work without it.
Share previews, one page at a time
Squarespace writes a share preview for every page from its SEO title and description, so the first job is those two fields, which the titles guide for Squarespace covers. For the picture:
Open the Pages panel, hover over the page and click the gear icon beside its name.
Social Image, then upload a photo of your work to Alternate Social Sharing Image. It is used only when that page is shared.
Only if the scan still names the page: Advanced, then paste that page’s block into Page Header Code Injection. One block per page, in that page’s own box, never the site-wide Header box, which would give every page the same card.
Business details
Squarespace keeps your phone number, address and business hours under Settings, then Selling, then Business Information, and shows them in places your template chooses. Its help centre does not say those details reach the structured data it writes, and on the sites we scan they often do not. That gap is what the business details file fills.
Fill every capitalised slot, or delete that line. Use the phone number and opening hours exactly as they appear in Business Information.
Paste it into the site-wide Header box under Code Injection, beneath your structured data, and save.
The viewport tag
Nothing to do. Every Squarespace template is responsive and serves a viewport tag, so a readability finding on a Squarespace site is about something else. The technical detail on the finding says which.
The footer contact block
Edit the footer in the site editor and add a text block with your business name, phone number and the towns you cover. Type it as text rather than pasting the HTML block, and rather than using an image. A machine cannot read a phone number in a picture.
Checking it worked
Load your homepage, view source and search for application/ld+json, then run the address through Google's Rich Results Test. Squarespace adds some structured data of its own, so you may see more than you pasted. That is fine and it does not conflict.
If your screens do not look like this, the wording has moved rather than the setting. Search your platform for the words in bold above. If you still cannot find it, send us the scan link and we will tell you where it is on your setup.