RestSharp post request - Body with x-www-form-urlencoded values

this working for me, it was generator from postman

        var token = new TokenValidation()
        {
               app_id = CloudConfigurationManager.GetSetting("appId"),
               secret = CloudConfigurationManager.GetSetting("secret"),
               grant_type = CloudConfigurationManager.GetSetting("grant_type"),
               Username = CloudConfigurationManager.GetSetting("Username"),
               Password = CloudConfigurationManager.GetSetting("Password"),
        };

        var client = new RestClient($"{xxx}{tokenEndPoint}");
        var request = new RestRequest(Method.POST);
        request.AddHeader("content-type", "application/x-www-form-urlencoded");
        request.AddParameter("application/x-www-form-urlencoded", $"app_id={token.app_id}&secret={token.secret}&grant_type={token.grant_type}&Username={token.Username}&Password={token.Password}", ParameterType.RequestBody);
        IRestResponse response = client.Execute(request);

        if (response.StatusCode != HttpStatusCode.OK)
        {
            Console.WriteLine("Access Token cannot obtain, process terminate");
            return null;
        }

        var tokenResponse = JsonConvert.DeserializeObject<TokenValidationResponse>(response.Content);

I personally find this way to work better for me when sending Form-UrlEncoded data.

public void GetResponse() {
        var client = new RestClient("api-url-here");
        var req = new RestRequest("endpoint-here",Method.POST);
        var config = new ClientConfig();//values to pass in request

        // Content type is not required when adding parameters this way
        // This will also automatically UrlEncode the values
        req.AddParameter("client_id",config.client_id, ParameterType.GetOrPost);
        req.AddParameter("grant_type",config.grant_type, ParameterType.GetOrPost);
        req.AddParameter("client_secret",config.client_secret, ParameterType.GetOrPost);
        req.AddParameter("scope",config.scope, ParameterType.GetOrPost);
        req.AddParameter("response_type",config.response_type, ParameterType.GetOrPost);

        var res = client.Execute(req);
        return;
}

Details on this parameter type can be found here: https://github.com/restsharp/RestSharp/wiki/ParameterTypes-for-RestRequest#getorpost