client = new RestSharp.RestClient("...");
                client.Proxy = proxy;

                var request = new RestSharp.RestRequest("/test",RestSharp.Method.POST);
                
                request.AddHeader("Accept", "text/plain");
                //request.AddHeader("Content-Type", "application/json");
                request.AddHeader("Content-Type", "text/plain");

                request.AddParameter("username", "username", RestSharp.ParameterType.RequestBody);
                request.AddParameter("password", "password", 
                //request.AddParameter("application/json", "accessToken", RestSharp.ParameterType.RequestBody);
        
                var response = client.Execute(request);

var request = new RestRequest
{
    Resource = "/test",
    RequestFormat = DataFormat.Json,
    Method = Method.POST
};
request.AddBody(new Auth { Username = "username", Password ="password" });