Update: between 2018 to 2019, Weather Underground (WU) was acquired by IBM, changing their API service and the process to apply for an API key. At the moment of this writing, the OpenSprinkler weather service by default uses Apple WeatherKit API and no longer depends on WU. From firmware 2.1.9, we have added back support for WU PWS (Personal Weather Station). Below we document the current way (as of July 16, 2019) to acquire a WU API key and PWS station name. We do not know how long this will be valid till they make changes again, so please use the instructions below at your own discretion.
Step 1: Create your WU account
Go to https://www.wunderground.com/signup and create a WU account.
Step 2: Create a PWS station
Log in to WU, then click on 'My Profile' at the top-right side of the screen, click on the 'My Device' tab. Here you will need to add a PWS (Personal Weather Station) device. You do not actually need to own a device physically. Enter your address, and from the list of available devices, you can select any (e.g. Acurite 3-in-1 Weather Station). It will generate a PWS station name. Please note that unless if you have a physical PWS that you own, this particular PWS will not have any valid data, so do NOT use this particular PWS for getting weather data.
Step 3: Create API key
Step 4: Finding PWS stations near you
From firmware 2.1.9, support for using WU PWS as data source has been added. Once you have the WU API key, you can input that in Edit Options -> Advanced. The submit. Next, go to Edit Options -> System, click on Location, it should show you nearby PWS stations in blue dots. You can select any of those blue dots, and submit. It will automatically record the PWS station name. Note that at the time of this writing, if you choose PWS as data source, you cannot choose ET as the weather adjustment method, because WU PWS data lacks 'solar radiation' parameter required by ET. Other weather adjustment methods, such as Zimmerman, can work with WU PWS.