How to Solve this Error

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C#

var client = new RestClient("http://example.com");

var request = new RestRequest("resource/{id}", Method.POST);
request.AddParameter("username", txtUsername.Text); // adds to POST or URL querystring based on Method
request.AddParameter("password", txtPassword.Text); // adds to POST or URL querystring based on Method

// execute the request
RestResponse response = client.Execute(request);
var content = response.Content; // raw content as string

This Code Showing Error

Error 1 The type or namespace name 'RestRequest' could not be found (are you missing a using directive or an assembly reference?)

Error 2 The type or namespace name 'RestClient' could not be found (are you missing a using directive or an assembly reference?)