@joephoto, thanks for reaching out and joining the Rachio community.
Wireless rain sensors have to double up a wire on the "SC" terminal in Gen2 -- this support article has a good visual to reference.
For the Hydro-Rain HRC-100 Wireless Rain/Freeze Sensor, I'd recommend the following wiring setup:
Red 1 - SC
Red 2 - SP
White - SC
Yellow - S1
Don't forget to enable your rain sensor after wiring it up. The support article above references the steps to do so
Hope this helps.