TurboTax downloading from CRA's Auto-Fill My Return service when you sign into your My Account from inside TurboTax.
When you sign in from in TurboTax, TurboTax enters your SIN number into the window for you. All you need to do is enter your My Account sign-in credentials and then TurboTax takes over.
If you used TurboTax Online, sign in to TurboTax Online for the tax year 2017 here: https://ww1.2017.turbotaxonline.intuit.ca/. If you used TurboTax CD/Download and you saved it on your computer you can simply go to the Start button on the bottom left of your screen and click on Start and you will see a small search bar - type in .tt17 and you will find it there. If you did not save it, unfortunately, there is not a way to access it.