Typically just after posting I found this page in the docs that mention you need to change grant_type
in order to use the realm parameter.
At first (on speed reading) I tried tho use "grant_type": "password-realm"
but received an error:
{
"error": "unauthorized_client",
"error_description": "Grant type 'password-realm' not allowed for the client.",
"error_uri": "https://auth0.com/docs/clients/client-grant-types"
}
Checking the error URI and back to the original page, I realised that grant_type should be the fully qualified URI:
http://auth0.com/oauth/grant-type/password-realm
And now this is working successfully.