under "URL construction" it gives you an example of a station ID being used.
I just tried to get the current conditions of my own PWS in JSON format according the instructions in the doc linked above by Victoria and got the result: "CDN-0001","message":"Invalid apiKey."
Does it it mean my old API key is not valid anymore for V2? Can I get a new one or it's a dead end?
The API for forecasts doesn’t include station ID as one of the options for specifying location in the URL. See https://goo.gl/QBN6Rc.
This is different than the requests for current conditions and history. I suppose it makes sense, since the forecast is for an area, whereas history and conditions are specific to a particular station. However, it is somewhat inconvenient since it means you need to make two API calls: one to get the location of a station, and another to get the forecast. In practice for me it’s not a big deal because I can use my station’s latitude/longitude rather than its ID when getting a forecast.