Hi @Pedro_Pereira,
First of all, please don’t ever expose your api_key or other people might use it!
I just tested this out shortly and @HendrikLeuschner is right.
You need to pass:
Entity<String> payload = Entity.json(
"{\"locations\":[[9.70093,48.477473],[9.207916,49.153868],[37.573242,55.801281],[115.663757,38.106467]]}"
);
and everything will work without a problem.
Please read 403 Forbidden - Daily quota reached or API key unauthorized
for further info on the escape strings.
Best regards