It will show Domain Not Claimed This domain has been mapped to Squarespace, but a website has not yet claimed it. If this is your domain, claim it in the Domains tab of your Website Manager.
now create an account at Squarespace, create a website template and then go to settings and click on domain and use a domain I own
Squarespace Subdomain Takeover on EdgeCase
Proof
Documentation
the reference article is https://hackerone.com/reports/1717626