Solution
In Canvas, navigate to your Assignments tab and click the + Assignment button.
Select the External Tool as your submission type and choose the Big Ideas Math app.
Choose the Assignments option in the categories that appear in the pop-up.
Navigate to the Assignments tab within Big Ideas Math and select the assignment that you would like to add a student to.
Click on + Add Item at the top. Confirm the action in the pop-up that appears.
This will create a new link for this assignment in Canvas.
Back in Canvas, launch the Big Ideas Math app from your left navigation bar and go to your Assignments tab.
Select that assignment you've just linked and click Assignment Details.
Select that student you would like to add to it and then save your changes.
Note: To reflect grades in the Canvas gradebook, users will need to launch the assignment or report from Canvas first.