Play for Scala: CORS error

Trying to implement a CORs filter to handle GET and POST requests kept failing. The error returned on each POST:

No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3003' is therefore not allowed access. apiservice.js:250 XMLHttpRequest cannot load http://localhost:9000/api/endpoing. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3003' is therefore not allowed access.

After triple checking that the Scala setup was right, I double checked the actual service sending the request. Apparently you have to explicitly JSON.stringify your data, other ways it gets send as a query string, and Play won't be able to parse it correctly.