You can select "Cheating Mitigation" in the Custom Test Builder (or when editing a custom test).
Cheating mitigation enables some basic tools for keeping applicants from referencing outside materials:
The test will play a warning sound and display a pop-up if the test taker leaves the test window. This will ask them to keep the test open and warn them that if they leave the test window again, it will automatically submit.
The test report will show you timestamps for every time the applicant left the test window, as well as any warnings they received and whether the test was auto-submitted.
The Cheating Mitigation tool is a great step towards ensuring a fair, transparent testing process. Of course, there are still ways applicants could circumvent the system, but in general, these common-sense measures are enough to both clarify your expectations about outside materials and prevent many instances of cheating.
For more context, see our complete walk-through of the Custom Test Builder.