If you enter the ctrl+alt+del command while using Synergy, it will only register on the server computer no matter which computer is in focus.
In order to send the to use ctrl+alt+del on your client computer, you will need to setup a hotkey.
You can do this by going into the "Configure server" menu, then selecting the Hotkeys tab.
Here you can select a key you wish to press, for example, "F12", then determine what this keypress should do. Please see the screenshot below for reference.
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