The T2125 form is broken down into multiple screens of questions inside TurboTax under the "Self-employment" section. You can go to Self-employment by either finding "Self-employment" card on your Tax Home, or by searching "T2125" or "Self-employment" in the search bar.

In the Self-employment steps, you will first answer a few questions on basic info, income, then you will arrive at "Select the types of expense you have", where you can add "home office expense". Select "Add", continue to next step then click "Start" the Home office expense on the next page.