RestClient' does not contain a definition for 'Execute'

Question

I have been working on getting an api authentication to work and I am currently stuck on getting the response to execute to show up in a text box.

{
    private void button1_Click(object sender, EventArgs e)
    {
        string user = textBoxUsername.Text;
        string password = textBoxPassword.Text;
        string tenant = comboBoxCompany.Text;
        var client = new RestClient("https://server_name/v2/security/authenticate");
        var request = new RestRequest();
        request.Method = Method.Post;

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

        request.AddParameter("application/x-www-form-urlencoded",
        "grantType=password&userName={{username}}&password={{password}}&tenant={{company_name}}", ParameterType.RequestBody);

        IRestResponse response = client.Execute(request); <= //error here // red line is under the 'Execute'

        textBoxJson.Text = response.ToString();


    }
}
public class IRestResponse
{
    public bool success { get; set; }
    public List<AuthenticationResponseData> result { get; set; }
}
public class AuthenticationResponseData
{
    public string accesstoken { get; set; }
    public string refreshtoken { get; set; }
    public string accessTokenExpiresOn { get; set; }
    public string refreshTokenExpiresOn { get; set; }        
    }

This is within a form page.

Answer 1

var request = new RestSharp.RestRequest("RESOURCE", RestSharp.Method.POST) { RequestFormat = RestSharp.DataFormat.Json } .AddBody(BODY);

var response = Client.Execute(request);

// Handle response errors
HandleResponseErrors(response);

from How to POST request using RestSharp