Unfortunately the keyboard layout not being set correctly on the login screen is a known issue.
You can probably type this on the login screen as follows:
Hold Ctrl + Alt and press u, then release Ctrl and Alt
Type f then 6
Press space to accept the character
The other way you can work around this is to try using an incorrect password a couple of times – this will trigger a password-reset flow.
Once you have managed to get back into your account, you can follow the steps on Can't switch to AZERTY layout on login screen - #2 by wjt to work around the problem. Assuming you use a standard German keyboard the incantation you want is: