To claim the benefits from the treaty you will need to attach Form 8833 to your returns.
Form 8833 is not supported by the TurboTax software. To be able to use the treaty exemption you will need to download and complete the form from the IRS for each of the tax years.
If you have not already filed your 2017 or 2018 returns you are unable to electronically file the returns at this time and must mail in your returns. You will can use the TurboTax desktop software. To download the software packages to your computer you can go to 2017 & 2018 TurboTax software.
When preparing your returns you would enter the amount of wages, you are exempting, as a negative amount in the Miscellaneous Income section of the return. You should use the description "Exemption per Treaty". After you print the returns attach Form 8833 to the return before you mail in the returns.
If you have already filed the returns for 2017 and 2018 you will need to amend your returns and enter the entry in the Miscellaneous Income section. You would attach Form 8833 to the Amended Return when you mail it in.