C# StringBuilder to registration_ids FCM Запрос в C# как параметр RestSharp

Question

        var client = new RestClient("https://fcm.googleapis.com/fcm/send");
        client.Timeout = -1;
        var request = new RestRequest(Method.POST);
        request.AddHeader("Authorization", "key=AAAAkRIqC40:APA91bF_GBeh22Me4HR2xOPNMnbhqgsIzW5xhF1dF00DjmM1xq42x33s2P7uz_EGrGZfX64mhId-s0B9LJyqcAHwI8enjsdvUPzlgQ_IMvFHpik7N53g0zN8KyaVPa9WzvFGmDYvP_4q");
        request.AddHeader("Content-Type", "application/json");
        request.AddParameter("application/json", "{\n    \"notification\": {\n        \"title\": \"New Arrival !\",\n        \"body\": \" Product : "+ txtItem.Text +" for : "+ txtPrice.Text + "\",\n        \"image\": \"https://lh3.googleusercontent.com/ddlJ846HYK5_qn2Ty8nl1QvgH3IPeGWAzekBcJXL05KNhsuKF9d2fUdt-Kag8rS7Ga8=s180-rw\",\n        \"click_action\": \"FLUTTER_NOTIFICATION_CLICK\"\n    },\n    \"data\": {\n        \"click_action\": \"FLUTTER_NOTIFICATION_CLICK\",\n        \"sound\": \"default\",\n        \"status\": \"done\",\n        \"screen\": \"new_arrivals\"\n    },\n    \"registration_ids\": [\""+ tokens.ToString() +"]\"\n}", ParameterType.RequestBody);
        IRestResponse response = client.Execute(request);
        string s = response.StatusCode.ToString();

Это мой код токена StringBuilder. Это может быть массив тоже хорошо. Но это возвращение Bad Request