How to send CTRL-ALT-DEL?

How can I send CTRL-ALT-DEL from the staff Application to the client application without the client OS intercepting it?
The client screen locked and I did not find a way to send the unlock keys.
I tried this [1], but it did not work.

[1] https://www.pointdev.com/en/faq/faq-ideal-administration-send-ctrl-alt-del-tightvnc-vnc-logon-sas-computer-machine-id-227.html