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In the resulting sidebar, click the plus icon next to 'Quickly Attendance 2019+'. This will add the tool to the bottom of your course feed in a hidden status; if you intend for your students to use the tool to check themselves in for attendance, use the dropdown below the tool title to make it visible to students.
Once the link appears, you can click on it to begin the setup process. The setup process will open in a new tab on your browser.
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