@marekd
I'm using Meteobridge to provide my data and had the same problem; how do I know if I'm using the v2 API? Below is how I updated it, but the steps should work whatever software you use;

1. I made sure Meteobridge was updated to the latest version. This changed the available configuration for Windy, a new Station ID was added and the API key was replaced with password.
2. Went to my weather station on Windy https://stations.windy.com/stations
3. Copied the password and pasted over the original API key in Meteobridge
4. Copy the station ID and paste into Meteobridge
5. Checked that it was successfully uploading. In Meteobridge it shows success or an error; I'm sure other software is similar. I also checked that the Windy website showed success as @marekd described above ("show recent requests")

Hope this helps people