How To Wire Humidistat and Humidifier - Honeywell
It goes &quot;HUM -&gt; U, other U-&gt;solenoid terminal, other solenoid terminal-&gt;C&quot;
In order to get your humidifier to work, you need to create a closed circuit through the humidifier solenoid to bring 24VAC to it.  Since we know the HUM 24V terminal is a 24VAC source from the wiring diagram you posted with the C terminal as its corresponding return, we can pull 24VAC from that and feed it to one U terminal on the humidistat.  The other U terminal, then, connects down to one of the humidifier's solenoid connections; finally, the remaining terminal on the humidifier solenoid gets connected to the C wire on your furnace.  The result is a closed circuit that only is energized when your furnace permits humidification and the humidistat calls for water, just as you wish it to work.