You probably arrived here by following the getting started tutorial for utopia at Utopia: Tutorial: Getting started.
Try sending some POST requests to /echo to test echo'ing back your response:
$ curl -d marco https://gjonasts-playground.playground.gocardless.dev/echo marco
This is just some random text
MYENV=blee
ENVIRONMENT=playground
MYSECRET=supersecret
MYOTHERSECRET=supersecret