You can’t attach a letter or any supporting documentation to your tax return. The CRA’s NETFILE system (and Revenu Québec’s NetFile system) aren’t designed to accept supporting documents.
We recommend filing your tax return online and sending supporting documentation only if requested by the CRA or Revenu Québec. For the most part, they do not need (or want) these documents.
If you need to file an election with your tax return, however, and you can’t find that election in Wealthsimple Tax, you’ll need to print and mail your return. Include a cover letter stating you are filing that election with your return.