Hi I already filed my tax return with some other service, but i want to file T1135 using turbotax, is it possible to do so? Does it allow standalone submission of T1135?

You can login to TurboTax Online, open your return and then search for “T1135” on the Tax Home page (2024 tax return on the left). Once you've filled out the T1135, you can go back to the Tax Home page and click on "Finish & File". You'll have to go through the review again, but when you get back to the Filing page, you should see an option to file just the T1135.