Signal extraction from Float Switch without disturbing existing circuit
I suspect that this is an AC system, running at mains voltage; kitchen appliances tend to be simple. However, it may be a modern design with a processor and display.

If it is a DC circuit, use an opto-coupler; find the polarity and connect accordingly. If it is AC, reverse connect a diode across the opto input. You may need to buffer the output in order to drive a relay. If the unknown circuit is a high impedance input then this won't work, but you could try sensing a change of voltage across the sensor. Either way you need an auxiliary power supply, or a detailed schematic to use the processor supply if it exists.