Unfortunately, as you note, TurboTax doesn’t support Form 8621, so you’ll need to file it through the mail.

It sounds as if you’ve already electronically-filed your return for which Form 8621 applies.  If that’s the case, you’ll need to amend your original return, attach Form 8621, and file them on paper.  See “Where to File” in Instructions for Form 1040X, Amended U.S. Individual Income Tax Return for the address. 

If you haven’t filed your return yet, you’ll likewise need to file your original return with Form 8621 by mail.  You can find the address in “Where Do You File?” in Form 1040 Instructions.

[Edited 03/02/2020 12:54 pm]