Mavericks: Enable virtual keyboard on login screen
This answer focuses on the case where you want to access the virtual keyboard because of layout/external keyboard uncertainties.
If you only have access to a mouse/pad this answer will not help you.
There is a keyboard shortcut that propose you to access to Accessibility Options, and it is working at login screen. (Only on High Sierra.)
Opt-Cmd-F5
And for MacbookPro w/ Touch Bar:
fn-Opt-Cmd-F5 or Triple click on TouchID button
From this new window, you can click and enable the virtual keyboard:
Here is the Mac accessibility shortcuts ref from Apple.
EDIT 2: for macOS Mojave
If the problem is that you're not sure about your keyboard layout, try this:
Opt-Enter : It lets you enter the login in a clear text field. Use it to visually check your layout \o/
EDIT: (added information from comments)
- Only available on High Sierra
- Shortcut for MacbookPro with Touch Bar
I just want to expand on what Ivan has said,
According to macOS User Guide, the Turn on accessibility options in the login window on Mac option seams to be available only on macOS Mojave 10.14
I have followed the Guide and it works as expected. (I needed a way to login to my MBP since the built in keyboard is intermittently not working.)
I could not find it under Accessibility, however it can be found in System Preferences-> Users & Groups-> Login Options Accessibility Options. Once you have the keyboard checked it will be on the screen the next time you login. I am currently running 10.14.6
I am sorry to say that this is not possible in Mac OS X unlike in Microsoft Windows. Maybe you could just wait for Apple to include it in their future releases. I hope I helped you :-)
The answer by itMaxence regarding High Sierra doesn't seam to be valid any more (perhaps apple changed something in one of the updates).
According to macOS User Guide, the Turn on accessibility options in the login window on Mac option seams to be available only on macOS Mojave 10.14
I can confirm that the accessibility options/keyboard is not working on 10.13.4 High Sierra when the screen is locked (login screen).