The regular method of accessing the safe mode is by pressing F8 key while the Windows Operating System is loading after the BIOS screen is displayed. When using a USB connected keyboard, pressing F8 key may not put the Windows Operating System in safe mode. It is required to have activated support and compatibility depending on the hardware configuration and type of motherboard brand for the USB Device. You can enable the options for legacy support for all USB Devices in CMOS or USB peripherals Legacy Support. You may find this option under Integrated Peripherals and devices menu.
You normally access safe mode by pressing F8 before the OS starts to load at startup (just after displaying the BIOS screen).
But with a USB keyboard, sometimes it doesn't work.
It requires that you activate support for this device in the BIOS:
Depending on the brand of your Motherboard
Look for the option: USB Device Legacy Support or All USB peripherals in CMOS and set it to Enabled.
This option is usually in the Integrated Peripherals menu.