Instructors add Turnitin as an external app to an assignment. This video demonstrates the process (assuming you have enabled the custom codes in your LTI configuration for Turnitin or the steps are summarized as below:
Go to Canvas:
1. Click on Edit for an Assignment
2. Under Submission Type, choose "External Tool"
3. Click Search and scroll down to TurnitIn.
4. Check the box that opens in a new window.