Console.WriteLine(response.Data);

RestClient client = new RestClient("https://euterpe.webuntis.com/WebUntis/jsonrpc.do?school=HTL-Perg");

        public void login()
        {
            var request = new RestRequest(Method.POST);
            request.AddHeader("Content-type", "application/jason");

            request.AddJsonBody(new
            {
                id = "ID",
                method = "authenticate",
                @params = new
                {
                    user = "kung",
                    password = "foo",
                    client = "CLIENT"
                },
                jsonrpc = "2.0"
            });

            var response = this.client.Execute<AuthenticationResponse>(request);
            Console.WriteLine(response.Data);

{"jsonrpc":"2.0","id":"ID","result":{"sessionId":"A173BECFB75F66123D6B678BE9745A29","personType":5,"personId":1234,"klasseId":123}}

public class AuthenticationResponse
    {
        [DeserializeAs(Name = "id")]
        public int id { get; set; }
        [DeserializeAs(Name = "res")]
        AuthenticationResult res { get; set; }
    }

    public class AuthenticationResult
    {
        [DeserializeAs(Name = "sessionId")]
        string sessionId { get; set; }
        [DeserializeAs(Name = "personType")]
        int personType { get; set; }
        [DeserializeAs(Name = "personId")]
        int personId { get; set; }
        [DeserializeAs(Name = "klasseId")]
        int klasseId { get; set; }
    }