You probably arrived here by following the getting started tutorial for utopia.
Try sending some POST requests to /api/messages to store messages:
$ curl -X POST https://dgomes-playground.playground.gocardless.dev/api/messages \
-H "Content-Type: application/json" \
-d '{"user_id": 123, "message": "Hello world"}'
$ curl https://dgomes-playground.playground.gocardless.dev/api/messages?user_id=123