Cannot GET access tokens using Authorization code in .net console app, which work in POST MAN

Hi All, I like to seek help on a issue i am facing when i try to authenticate into BOX.

Step 1: i used https://account.box.com/api/oauth2/authorize?response_type=code&client_id="my id"

             to get Auth code.

Step 2: i tried to get Access & refresh tokens using postman it worked fine.

Step 3: Now i tried to automate process of getting Access and refresh tokens using .net code.

string param = string.Format("grant_type=authorization_code&code={0}&client_id={1}&client_secret={2}", "mycode", "myid", "mysecret");

var client = new RestClient("https://pfizer.app.box.com/oauth2/token/");

var request = new RestRequest(Method.POST);

request.AddHeader("content-type", "application/x-www-form-urlencoded");

request.AddParameter("application/x-www-form-urlencoded", param, ParameterType.RequestBody);

var response = client.Execute(request);

var json = JObject.Parse(response.Content);

Console.WriteLine(json);

I always get Bad request ,

{ "error": "invalid_grant", "error_description": "The authorization code has expired" }

I am not using auth code 2 times anywhere, even fresh token does not give any response.

Reference article:

https://stackoverflow.com/questions/40826251/get-an-access-token-from-box-api-from-asp-net-application?rq=1

Please help.

Thanks.