Rain Sensor Wiring

Hey guys, I wired up my Rachio 3 8 zone last week and it works great. I didn’t bother hooking up our existing rain sensor though, and now want to go back and connect it. Looking through the wiring guides, I’m not sure how to connect it though. It looks like it only has a red and black wired and I’m not certain where to connect them on the Rachio. Any help would be greatly appreciated. I’ve attached an image of the original setup below. Thank you!

Rain sensor one wire - doesn’t matter which one - to S1 and activate the Rain Sensor setting for that port in the Rachio App and the other wire in the 24 VAC- port.

I finally got around to wiring this up and now this keeps happening…what did I do wrong?

@JayKuehn - I didn’t see anything hooked up to the Weathermatic sensor ports (a wire was looped between the two posts). Is the device proprietary to Weathermatic? Can a picture of the device and the Rachio wiring be posted?