I’ve setup a free service, which you can use, at wufyi.com.
First check that WU station you are considering provides good data. More on this here (link).
Then you can setup wufyi.com to transfer data. More on that here (link). Sorry updated setup is not ready yet.
When setting up pwsweather station (which would make WU data available to rachio), be careful about GPS coordinates when setting up pws weather station. Here is a guide (link).
That is about it. Setup takes about 10 minutes and you don’t need anything else.
Cheers,
Gene