Humidistat but only while furnace is on

Hi all. I had to relocate my humidistat when I replaced my thermostat due to lack of wires, and no way to fish a new one. I’ve been unhappy with where it is now, but then it occurred to me, I have several humidity sensors, why not get HA to do it. So I looked, saw the generic hygrostat, set it up with an ESP-01 and a relay, and a Humidity card in HA. Great. But…
I don’t want the relay energized for hours on end if the furnace isn’t running. So I’d like a condition that says only if furnace=on or, furnace plenum temp greater than 29C. I don’t see any option in the generic hygrostat config, so maybe I need an automation. I’m not that great with automations, so I’m not sure what I’d use for the trigger, nor how to turn it off when the furnace turns off, or the humidity hits target. So I guess what I want to do is have the hygrostat call the automation instead of turning on the relay, the automation does an if furnace plenum > 29C turn on relay. But not sure how to do all that.
Any suggestions would be greatly appreciated. Yes I could wire it so that the relay board only has power when the furnace is on, but seems to me it would be more efficient to do it programattically.