Configuring a Uni Plus to trigger a humidifier from a Ecobee

I'm hoping to configure the Uni Plus to sit in the wall behind my Ecobee and detect a signal from the ACC+ output. I have a Ecobee set up to control a humidifier using the ACC+ and it seems to be working.

The Uni is connected with Red to Rc, Black to C and White to Acc+. The Uni does not detect the ACC+ when the thermostat is calling for humidity. Any idea what I'm doing wrong? This is my first time using Shelly so I'm probably doing something stupid but I can't find much useful on software configuration of the Uni. I suspect I have to tell it to look for the input then announce to the Homey Pro that is sees something.