It is possible to do it online instead of desktop software, but it might be more difficult. The desktop software would at least allow for Forms Mode to print/view/edit the forms during preparation to be sure they were exactly like the original.
Since the amendment is for 2018 (the current online year), it "could" be prepared in online TurboTax by recreating the 1040 Federal return as originally filed, then similar to the desktop product, indicating that one intended to file that "original" return by mail (but not actually mailing it.) Then TurboTax would think an original return was filed. Then one could start the amended return in Online TurboTax.
NOTE: You'll likely also have to amend a state return as well, if applicable. So you'd need to re-create the state return before you amend the Federal return.
