Hello,
Thank you for reaching out!
Unfortunately, "System events" only logs events, that are initiated from known sources in the system (reboot from SMS, WebUI, etc.), and adding them manually is not possible.
One solution to troubleshoot this issue would be to enable logging to flash memory. This would save the logs even after reboot and would most likely show the origin of the reboot. This could be configured by navigating to System → Administration → Troubleshoot and changing the "Save log in" option from RAM memory to Flash memory.
However, please only use this option if reboots happen often, as enabling this option can increase flash wear and may shorten the lifespan of the product.
Another solution would be to log these events in the external logging server. This can be set up by navigating to System → Administration → Troubleshoot and specifying the logging server IP, port, and protocol.
Let me know if any more information is needed!
Best regards,
DaumantasG