How to wire humidifier to furnace
It looks like the humidifier connections on the board will provide 24V for the humidifier when the furnace is on so I don't know that you need the transformer. Do you know which terminals on the board those are? If not post a photo of the board. I assume the humidistat only has two wires and the humidifier only has two wires, if so then the attached wiring diagram should work.