The extension request, Form 4868, will be included in the PDF of your return if you download the PDF to include All forms and worksheets.
If you have already paid the TurboTax account fees and are working within the 2018 tax return, click on My Account at the top of the online program screen. Then click on Print Center. Then click on Print, save or preview this year's return. On the TurboTax Print Center window, select from the dropdown Forms filed with the government plus all worksheets. Choose the option on how you want to print or save.