It’s a GET request but you’re putting the params in the request body, like in a POST request, while it should be just simple query parameters.
Note that there are Auth0 Postman collections out there, I think that’ll be the easiest to get started in Postman.
https://auth0.com/docs/api/authentication#code-samples
You can test the endpoints using either the Authentication API Debugger or our preconfigured Postman collection. For some endpoints, both options are available.