Demystifying iControl REST Part 6: Token-Based Authentication

Im new to REST API scripting so Im probably making a simple mistake here, but Im using Postman to generate a POST request to get a token. Im hitting .

I have authorization set to basic and in Body Im sending:

    {
        "username":"admin",
        "password":"password",
        "loginProvidername":"tmos"
    }

with the actual password for the admin account.

I keep getting:

    {
        "code": 401,
        "message": "Cannot use 'local' auth provider name when system  auth source type is not 'local'",
        "originalRequestBody": "{\"username\":\"admin\",\"generation\":0,\"lastUpdateMicros\":0}",
        "referer": "10.238.90.21",
        "restOperationId": 37917423,
        "kind": ":resterrorresponse"
    }

I've Googled myself cross-eyed and haven't come across anything helpful. We use RADIUS authentication for access to our F5s. Based on this error message I think that has something to do with this. It seems that it doesn't like me using a local account when the box is configured for RADIUS, but I can't figure out how to change the post to use RADIUS. I've tried using a known good RADIUS account and that fails with the same message. The local account works with basic authentication when doing a GET to something like /mgmt/tm/ltm, but not when I do a POST to the /mgmt/shared/authn/login.

I am testing this against version 13.1.0.8.