How do I wire my furnace humidifier so it comes on any time the fan is on?
You could use the EAC-1 terminal to power a step-down transformer. This is common with furnaces that energize the HUM terminal to line voltage.  

Check the humidifier manufacturer, they should have a properly sized  transformer to fit this exact need. They should also have a wiring diagram for this type of setup, in the installation instructions.

Basically, the primary side of the transformer will connect to the EAC-1 terminal, and a neutral terminal on the furnace control board. Then the secondary side will power the humidifier.  

Whenever EAC-1 is energized, it will power the transformer, and the humidifier will turn on.



As noted in the comments on the question, you may not get the expected results.