T1135 is not included in your personal income tax return. There is a special department to assess the form. Even you are filing a paper return, T1135 has to be mail in separately to a different address. So it is reasonable that CRA has decided to open a separate service to receive the form electronically.
To file, go to the same filing page "File Your Returns", click "File Now", you will see the button to file your T1135.