By enabling test mode

Open an elevated command prompt

bcdedit /set testsigning on

But don't forget to disable it again when you have installed the drive

bcdedit /set testsigning off

Also SecureBoot must be DISABLED

By disabling integrity checks

Open an elevated command prompt

bcdedit /set nointegritychecks off

But don't forget to enable it again when you have installed the driver

bcdedit /set nointegritychecks on

Also SecureBoot must be DISABLED

From advance boot menu

Restart your computer by holding the shift key while clicking on restart

In the advanced boot menu choose:

1. Troubleshoot
2. Advanced options
3. Start-Up Settings (may be on a second page!!)
4. Restart
5. Than choose F7 to disable driver signature enforcement

After the reboot  you can install unsigned drivers. When you have installed the needed driver reboot again to enable driver signing again.