something like this?
private static IRestResponse<Orcid> GetAuthorisationResponse(string code)
{
var clientId = ConfigurationManager.AppSettings.Get("ClientId");
var clientSecret = ConfigurationManager.AppSettings.Get("ClientSecret");
var redirect_uri = ConfigurationManager.AppSettings.Get("RedirectUri") + "/orcid.aspx";
var client = new RestClient("https://orcid.org/oauth/token");
var request = new RestRequest(Method.POST);
request.AddHeader("Accept", "application/json");
request.AddHeader("Content-type", "application/x-www-form-urlencoded");
request.AddParameter("client_id", clientId);
request.AddParameter("client_secret", clientSecret);
request.AddParameter("scope", "/read-public");
request.AddParameter("grant_type", "authorization_code");
request.AddParameter("code", code);
request.AddParameter("redirect_uri", redirect_uri);
IRestResponse<Orcid> response = client.Execute<Orcid>(request);
return response;
}