This is what the website says:
How to purchase On My Way:
The On My Way service can only be purchased on aircanada.com 96 hours or more before your flight. Here’s how:
At time of booking:
Select the On My Way option when choosing travel options after you have made your flight selection.
If you have already completed your booking:
Go to the Manage my Bookings page, retrieve your flight booking, and you’ll have the option to add the On My Way service at any time 96 hours or more before your flight.