Setting safe mode to the next reboot has become quite tricky.
Usually you have to navigate among one-way system options (reset, restore, …).
Recently I discovered that you can easily trigger it from Command Prompt with administrator access rights.
Type the following command:
bcdedit /set safeboot minimal
The system will reboot in safe mode as requested this time only.
If it will not switch back to normal mode by itself, you can help it by this other command:
bcdedit /deletevalue safeboot
and reboot again.