You do not need to file on 02/2819 to get the discount.  You just need to pay the TurboTax account fees before the end of 02/28/19 to get the discount.  Then complete and file your tax return at any time.

To pay the TurboTax online account fees by credit card, without completing the 2018 return at this time, click on Tax Tools on the left side of the online program screen.  Then click on Print Center.  Then click on Print, save or preview this year's return.  On the next page, to pay by credit card, click Pay Now.  On the next screen just click on the Continue button.  The next screen will show a review of your Fees, click on Continue.  Complete the next screen to pay by credit card and click Continue.