Just did the Facebook domain verification, here are the steps:
In FB Business Manager, add you domain in Business Settings>Brand Safety>Domains.
I chose the DNS method, click to copy the necessary information to the clipboard.
In Squarespace: Settings>Domains>Choose correct domain (if hosting more than one)
Advanced settings> Custom Records 'Add' >Paste needful>change Record to TXT and Save
Back in FB Business Manager - I waited only a few minutes, clicked Verify and it's done.