8621 is not handled by TT.
prepare your return in TurboTax. Fill out Form 8621 manually, and then insert the completed Form 8621 into their tax return documents (using Attachment Sequence Number 69). Finally, they would then paper-file their complete tax return, including Form 8621. This tax return filing would be sent to the normal, regular, IRS office where taxpayers in that person's particular state routinely file.
