Google Search Console

is a web service by Google which allows webmasters to check indexing status and optimize visibility of their websites.

This console is where you can view the performance of your domain, as well as verify ownership of your domains

1.Go to GSC

Run a quick search for Google Search Console

This will bring you to the console

Click on Add Property

This ensures all future subdomains will be automatically verified as well

Copy the blurred out part (it is unique for each site) and create a custom record (step 4 in Squarespace)

After creating the record, click verify, if it does not verify immediately, give the DNS some time to propgate

Once you see this popup, it means your domain ownership is verified. Now you can create subdomains for your google sites page

Go on to Google Sites tab to see how you can set the custom url