You can use the path used in the DarkSky API when access the API via Tyk too.
For example, if you define your API upstream target as https://api.darksky.net/, and have an API slug as weather, then you can access it through the Tyk Cloud as http://<your-org>.cloud.tyk.io/weather/forecast/37.8267,-122.4233.
Tyk will identify the API by the weather element of the path, then take the remaining of the path forecast/37.8267,-122.4233 and add that to the target URL to make https://api.darksky.net/forecast/37.8267,-122.4233.