I don't understand how to sent this json in the request body. Here is the json from the postman create collection body (https://www.postman.com/postman/workspace/postman-public-workspace/request/12959542-049042b8-447f-4f71-8b79-ae978cf40a04):

{
    "collection": {
        "info": {
            "name": "Sample Collection 909",
            "description": "This is just a sample collection.",
            "schema": "https://schema.getpostman.com/json/collection/v2.1.0/collection.json"
        },
        "item": [
            {
                "name": "This is a folder",
                "item": [
                    {
                        "name": "Sample POST Request",
                        "request": {
                            "url": "https://postman-echo.com/post",
                            "method": "POST",
                            "header": [
                                {
                                    "key": "Content-Type",
                                    "value": "application/json"
                                }
                            ],
                            "body": {
                                "mode": "raw",
                                "raw": "{\"data\": \"123\"}"
                            },
                            "description": "This is a sample POST Request"
                        }
                    }
                ]
            },
            {
                "name": "Sample GET Request",
                "request": {
                    "url": "https://postman-echo/get",
                    "method": "GET",
                    "description": "This is a sample GET Request"
                }
            }
        ]
    }
}

Json above works fine using the Postman app, my attempt at it:

RestSharp.Settings.Init("collections", Method.Post);
RestSharp.Settings.AddHeader("X-Api-Key", "I hid my key");
RestSharp.Settings.restRequest.RequestFormat = DataFormat.Json;

RestSharp.Settings.restRequest.AddParameter("application/json; charset=utf-8", "{\n    \"collection\": {\n        \"info\": {\n            \"name\": \"Sample Collection 909\",\n            \"description\": \"This is just a sample collection.\",\n            \"schema\": \"https://schema.getpostman.com/json/collection/v2.1.0/collection.json\"\n        },\n        \"item\": [\n            {\n                \"name\": \"This is a folder\",\n                \"item\": [\n                    {\n                        \"name\": \"Sample POST Request\",\n                        \"request\": {\n                            \"url\": \"https://postman-echo.com/post\",\n                            \"method\": \"POST\",\n                            \"header\": [\n                                {\n                                    \"key\": \"Content-Type\",\n                                    \"value\": \"application/json\"\n                                }\n                            ],\n                            \"body\": {\n                                \"mode\": \"raw\",\n                                \"raw\": \"{\\\"data\\\": \\\"123\\\"}\"\n                            },\n                            \"description\": \"This is a sample POST Request\"\n                        }\n                    }\n                ]\n            },\n            {\n                \"name\": \"Sample GET Request\",\n                \"request\": {\n                    \"url\": \"https://postman-echo/get\",\n                    \"method\": \"GET\",\n                    \"description\": \"This is a sample GET Request\"\n                }\n            }\n        ]\n    }\n}",ParameterType.RequestBody);

var response = RestSharp.Settings.restClient.ExecuteAsync(RestSharp.Settings.restRequest).Result;

Console.WriteLine("Returned http status code is: " + response.StatusCode + " expected OK");

Assert.IsTrue((int)response.StatusCode == 200);