The cord that runs to your light contains two wires.
The cord switch interrupts one wire.
The second wire runs uninterrupted between wall plug and light.
The photo below is not the same switch as yours, but it shows how one wire is interrupted and the other wire is uninterrupted.
Which wire do you want to interrupt?
Look at the wall plug.
Is the plug polarized, where one blade of the plug is wider?
If the plug is polarized, then one blade is wide and the other is narrower.
Each blade of the plug feeds one of the two wires going to lamp.
Locate the narrower blade, and the wire that comes from that side of plug.
The narrow-blade wire is the wire you want to interrupt when installing the cord switch.
If the plug is not polarized, then choose one wire.
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