There is currently no option to add a "Logout" button. As a workaround, it is possible to enable a timeout of the Defender Web Management Portal to terminate idle sessions.
1. Open IIS Manager
2. Select the Defender site
3. Under IIS open “Authentication”
4. Click on “Forms Authentication” and click the action “Edit”
5. Edit the field for “Authentication Cookie time-out (in minutes)” to the value you need.
6. Restart IIS and test to ensure the setting works for you.
The steps above can also be verified with the below Microsoft Article:
Version 5.8.1 and above: the logout button is now available.