You need to set your content-type to application/json. But -d sends the Content-Type application/x-www-form-urlencoded, which is not accepted on Spring's side.
Looking at the curl man page, I think you can use -H:
-H "Content-Type: application/json"
Full example:
curl --header "Content-Type: application/json" \
--request POST \
--data '{"username":"xyz","password":"xyz"}' \
http://localhost:3000/api/login
(-H is short for --header, -d for --data)
Note that -request POST is optional if you use -d, as the -d flag implies a POST request.
On Windows, things are slightly different. See the comment thread.
