适用于ARXIVAR IXFE的OAuth2

i'm actualy trying to use the OAuth2 api to authenticate to the IXFE api service of Arxivar. I'm using those link now as documentation: https://ixsupport.arxivar.it/File/Get/66299 is the documentation of my api https://ixapidemo.arxivar.it/oauth/swagger/ui/index#/ and this is the "online simulator"

Those are the data that i'm tryin to use, put it on the "request" filed to try (but u need username and password)

{ 
 "client_id": "FA7CD29F-F7CE-4BE6-87C1-8FAA51A5DA7F",
 "client_secret": "clientSecret", 
 "grant_type": "password", 
 "username": "********", 
 "password": "********", 
 "client_version": "v2.0.3.1"
}

I'm searching for someone that may have used those api before to let me know what am i doing wrong. Actualy i've found this page https://ixsupport.arxivar.it/KB/View/44886-clientid-e-clientsecret-per-accesso-alle-api-di-ixfe-v that says explicitly that the fields client_id and client_secret are not required for demo uri.

The error that these api return me is a general error

{
  "error": "invalid_request",
  "error_description": null,
  "error_uri": null
}

Someone have already used those api and can help me? Pls i'm not understending my mistake and the documentation is not so clear.