Please go back to File>CRA Online Mail and click Continue until you get to the “Filing your return(s)” page. There is a separate button for filing the T1135.
Go to My Info>Personal Tax Profile for [Your Name], and click Edit next to Your home and other property. Make sure that you have answered YES to the 3rd question "[Your Name], did you own or hold specified where the total cost amount of all such property, at any time in 2018, was more than $100,000 CAD?" foreign property"
If the button still isn't appearing, please contact our phone support as they will be better able to assist you: https://turbotax.community.intuit.ca/replies/3152013