When using SpeedGrader, you can enable Turnitin's Similarity report to check any student submission for plagiarism. The setting can only applied to File Uploads and Text Entry submission types.

Go to Assignments and click on +Assignments
Fill in the relevant details, select Online as the submission type to choose SpeedGrader
Once you select File Uploads or Text Entry, a new setting box will appear called Plagiarism Review - this will default as None, click this drop down menu and choose Turnitin

Turnitin similarity report set up in Canvas

4. Choose the similarity report settings of what you want the submitted work to be checked against or what you want to exclude.

5. When you scroll down, you can choose how students receive the similarity report.

Speedgrader setting of how students receive the similarity report

6. Continue to complete your assignment settings and click Save or Save & Publish

Once your students begin to submit their work, the report will be generated alongside the submission. You'll be able to view the report either in the SpeedGrader marking environment or Grades area of the course. Instructions on how to view the report are here: https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-view-plagiarism-assignments-in-SpeedGrader/ta-p/852