If the IRS only asked for Form 8606, I would send the only Form 8606, in this case with Part III properly prepared to show that your distribution is a nontaxable distribution of Roth IRA contribution basis. (With line 22 grater than or equal to line 19, line 25a will be zero.) Form 8606 has a signature block where you can sign and date the form.
If you will be continuing to use TurboTax in the future, it would be useful to used TurboTax to prepare From 8606 by going through the process of amending your filed tax return. This will allow TurboTax to track your Roth IRA contribution basis in case you make any future nonqualified distributions from your Roth IRAs. When going through the amendment process, first make sure that you have entered any Roth IRA contribution that you made for 2018, then enter the Form 1099-R that you failed to enter originally, answer the immediate follow-up questions to indicate that "you did something else with the money" (you didn't roll it over back to a Roth IRA). Click the Continue button on the Your 1099-R Entries page and answer the questions regarding your net Roth IRA contribution basis from years prior to 2018. TurboTax will use this information to prepare Form 8606 that you can print, sign and mail as instructed in the letter that you received from the IRS.
