C# JSON POST request to server always fails with "405 method not allowed"

I am trying to send a JSON to a authentication server. It supports POST requests but it does not work. This is currently my code:

 Using client = New WebClient()
        client.Headers(HttpRequestHeader.ProxyAuthorization) = ""
        client.Headers(HttpRequestHeader.ContentType) = "application/json"
        result = client.UploadString("authserver link here", "POST", json)
 End Using

The error it returns is "405 Method not allowed". What is causing that issue? My JSON is properly formatted, and the server works.