If you have to have a windows vm on board of your mac using parallels desktop and you have to have a user logon on startup of the machine let’s say because of your companies security settings you might have run into this problem earlier. Windows unlock screen request to ctrl+alt+del for login from you which does not work out of the box with your apple keyboard. In parallels you have 2 possibilities:
A) go out of full screen mode of your vm and select the keyboard icon at the bottom of the vm status bar and the keyboard shortcut menu will come up where you can select ctr+alt+del for login (see image).
B) you can use the short method by simply pressing the following keys on your keyboard:
Ctrl + ⌥ + fn + ⌫
(means: Ctrl + alt + fn + backspace)
this works because the fn key changes the functionality of the backspace key to a delete key.