When I hooked up my rain sensor I was told it can only be activated per the app.
Here's what I was told;
"Remember the rain sensor must be enabled within the Rachio App. To activate, simply log into the Rachio app (mobile only) and complete the following steps:
Device Settings > Advanced > Enable Rain Sensor Toggle".
This was per Rachio support when I asked about hooking up my Irritrol wireless rain sensor.
Hope this helps..