400 Bad Request - Access denied [C#, HttpWebRequest]

Thanks Benjamin for your help, now I receive a different response with different error code (400):
{
"error": "invalid_client"
"error_description": "Access Denied."
}

the credentials are the ones provided by my Dashboard after the login..

That error usually means that the User credentials you are using do not belong to the same application/account (which is required during the initial stages of development).

Check that the username, extension, and password, are all part of a User/Extension which is enabled and defined in your Sandbox account in the [Sandbox version] of the Online Account Portal: https://service.devtest.ringcentral.com