получение ошибки "unsupported_grant_type" при вызове Xero refre sh token api

Question

Я работаю с Xero Api для получения данных счетов. когда срок действия моего токена доступа истекает, я пытаюсь сгенерировать новый с помощью токена refre sh, но в качестве ответа получаем unsupported_grant_type.

Ниже приведен мой запрос токена Refre sh

var client = new RestClient("https://identity.xero.com/connect/token?=");
            client.Timeout = -1;
            var request = new RestRequest(Method.POST);
            request.AddHeader("grant_type", "refresh_token");
           // request.AddParameter("grant_type", "authorization_code");
            request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
            string plainText = "2C707E29C627444EA73AADB3F0E7448A" + ":" + "6iUQ0NyBBXpDpSKYaprvowUjXAif41uK1QHCtyBL3vArPotc";
            var plainTextBytes = Encoding.UTF8.GetBytes(plainText);
            string encodedText = Convert.ToBase64String(plainTextBytes);

            request.AddHeader("Authorization", "Basic "+ encodedText);
            request.AddHeader("Cookie", "Device=ac903639876241ba992f5170c176d7b7; _abck=12ACF4E93634D401FB7C7CD9B544788F~-1~YAAQDhwgFxmkCKxxAQAABPlOxwN07cMPzGAXcBjZYifvWgfzqqRd3uABVwuMX4OdtZPCU0OwcOkalVuQuFLUD3PNpJVe+bNkVZ45qWTjNW6gBhBiC2TUGZCQiSLekeP6HnbVkMZRZtepwUWgnTjCQ5/XWPJOchHM/ecE4rwA4xYMEy7emjMAolOGX8X5u70+R8xdVkZlj8YBk4NkeMglm6y1JVhTt/nq6SDGXBS34vAJD8T/68WZKCLVOSem/nJe8NUdShIjRWL+yRA2Pb1o+HobR3DJbsmlIe0ezy8SqgQAkLTBhdZjHw==~-1~-1~-1; ak_bmsc=FA12D3F371BC6D88443778C8FE9E2CB517D46377B84C00002455AA5EDBC7DA24~plR4FZN8ha50tsVyXSR+JkZ3uSvHbHVfTiQnq35HFaGEy2UHAuhq6dv5vGqV00GaFPoP70p2Dsf7Bc0aodRmJZcwU9N4vEcRrqbBVHV/5DOQF/eTLHXALoJDFc3esQx+Kz0EipR3u6eyG7sasDKtIiaOZp+xJ1Zx6SNQjeXzPrHE+3HszFaTVzRoblzkGcevuxAL7dsQEkrp4uB20CIU9OaXdjaHV4UaigOGlSsYo5nt4=; bm_sz=449A6535155E672AF783F7717001BF49~YAAQd2PUFzOmXshxAQAAG5VcyQfeizXKc2d1vZwMBqcVRnfmYnCrLBU2iaDaFofTIDWTr7FeK1nADuceBhikxVF3tXWeHSTOLkYULwIHuz7fioBZC7wPf03kJYMapENDooKz8n5z++a+s4N0JT2jXcZTVkdYDgbr6FG3ErOgZGTaGXbywmZs8Ze7pHPK; bm_sv=0D600B86F31575226013DBA1C988C3D0~9oZEcht4TxX7Vc9IjcjPiD/9Y4vNQqaHwgePlGWuiQ93Upjk156QtA/Yfv6WFvtxFQ5i7jO+EK/pFrb/1mdJpIlqbIHgamHN6mGCVsuFYE/vGbC5+pl9j3pPVtjv41yJjL4KnmUNSsbCT+JEhYIfEg==");
            request.AlwaysMultipartFormData = true;
            request.AddParameter("grant_type", "refresh_token");
            request.AddParameter("refresh_token", "04ed7e45fa36f0fc753257b0259a73c43e1effffbb8c588e00f0d5b262a39264");
            request.AddParameter("client_id", "2C707E29C627444EA73AADB3F0E7448A");
            request.AddParameter("client_secret", "6iUQ0NyBBXpDpSKYaprvowUjXAif41uK1QHCtyBL3vArPotc");

            IRestResponse response = client.Execute(request);
            Console.WriteLine(response.Content);
            dynamic data = JsonConvert.DeserializeObject(response.Content);
            AccessToken = data.access_token;