What EXACTLY is this function and how does one go about doing it? I'm not seeing anything in account setup . .
You're referring to linking to an Account in Quicken that you've hidden?
Typically you hide Accounts when you don't intend to use them anymore, so linking an account at a financial institution to a hidden Quicken Account wouldn't be done.
I've never tried to link a hidden Account to an external account and I don't know if it's actually possible. But assuming you know how to link an Account that isn't hidden, why can't you simply unhide it, link it, and then hide it again?