Dell P2223HC 21.5" 16:9 IPS Monitor - Manual (Page 38)

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Setting up the monitor
4. In the drop-down menu of When I press the power button , there are a few
options for selection, namely Do nothing/Sleep/Hibernate/Shut down .
You can select Sleep/Hibernate/Shut down .
NOTE: Do not select “Do nothing”, otherwise the monitor power button
will not be able to sync with PC system power state.