The type or namespace name "IRestResponse" could not be found (are you missing a using directive or an assembly reference?)

I have a problem with IRestResponse in the below:

public async Task<CezanneToken> GetAccessToken()
    {
        var client = new RestClient(WebConfigurationManager.AppSettings["TokenUrl"]);
        var request = new RestRequest();
        request.Method = Method.Post;
        request.AddHeader("cache-control", "no-cache");
        request.AddHeader("content-type", "application/x-www-form-urlencoded");
        request.AddParameter("application/x-www-form-urlencoded", "grant_type=client_credentials&client_id=" + WebConfigurationManager.AppSettings["ClientId"] + "&client_secret=" + WebConfigurationManager.AppSettings["ClientSecret"] + "", ParameterType.RequestBody);

        IRestResponse response = await client.ExecuteAsync(request);
        string serStatus = ((RestResponseBase)response).Content;
        CezanneToken details = JsonConvert.DeserializeObject<CezanneToken>(serStatus);
        string Token = details.access_token;

        return details;
    }

IRestResponse throws the The type or namespace name "IRestResponse" could not be found (are you missing a using directive or an assembly reference?) error and I cannot make it work. IntelliSense suggest to go with RestResponse instead of IRestResponse.

But when I go with RestResponse I get Bad Request on the response.

The code sample above is "translated" from Visual Basic but it works just fine in VB. I don't know if the problem with the Bad Request comes from using RestResponse but I assume IRestResponse is needed just as in VB.

I've also seen people using IRestResponse but it just doesn't work for me. I have the using RestSharp; but do I need something else as well?