After this change the equivalent would be:
curl -XPOST --jp username=YOUR.USERNAME password=YOUR.PASSWORD https://api.ctl.io/v2/authentication/login
Which isn't too far from your desired outcome (notably, without relying on argument position for meaning). Although, I guess the argument that "curl is already installed on almost every server", sorta gets moot because I imagine it will take a while for most distros to move to the latest curl that will support --json/--jp