I tried it in curl before your edit and yeah, "curl https://ext-sq.squarespace.com https://ext-sq.squarespace.com -H 'Host: cranberry-groundhog-8agy.squarespace

I tried it in curl before your edit and yeah, "curl https://ext-sq.squarespace.com https://ext-sq.squarespace.com -H 'Host: cranberry-groundhog-8agy.squarespace.com'" just returns this site. Probably something to use before your IT team sets up the CNAME record.