401 Unauthorized error on Users web service

I'm trying to POST a login request via Postman like this:

POST www.myurl.com/.../

with a body of JSON:

    "MembershipProvider": "OpenAccessProvider",
    "Password": "my-password",
    "Persistent": true,
    "UserName": "my-username"

But that gives me a 401 unauthorized error with a message of "You do not have permission to view this directory or page."

The route and parameters appear to be right based on what I'm seeing in the documentation at /Sitefinity/Services/Security/Users.svc/help. Any idea why I can't POST to that method?

@Jacob - did you get this resolved? I've got the same issue.