The float switch is placed in sump at the working water level. When the water evaporates the float switch drops and turns on the pump in the top-off water tank and fills the sump until the float switch floats up to the shut off point.
Here are 2 pics of a spare float switch I have. This first one shows the position the "float" would be in for the pump to be off. Meaning the water level is where you set it.
This image shows the float switch in the "PUMP ON" position meaning you have had water evaporate and the float switch is activated and turns your pump on.
You do not want a bulkhead connecting your FW
top off to your sump.