Configuring DNS for www without redirecting root.

Hello. I'm trying to configure the Squarespace to use an externally-administrated domain. Whenever I try to set it up with www.<domain>.com, Squarespace insists that I'm trying to migrate the root <domain>.com record. I do not want that. I just need the www.<domain>.com to point to Squarespace. In principle, I can run the migration, and everything goes through with root record pointing somewhere else, but Squarespace never considers migration finalized (because root A record doesn't point to Squarespace servers) and so the SSL certificates never get updated. Is there a way to fix that?

On that note, while I understand that a lot of people might enter www.<domain>.com when they mean <domain>.com, simply assuming this is always the case is exceptionally inconvenient.

I have the same issue with a corporate website. How can a deploy the website properly on www without impacting the root accesses?