CS0103 exception when trying to send SMS

Trying this code to send SMS in C# it is throwing a CS0103 at this point:

var response = await extension.Sms().Post(requestBody); //name extension does not exist in current context.

The full code
public async void sendSms()
{
RestClient rc;
rc = new RestClient("id, "clientsecret", "https://platform.devtest.ringcentral.com");
var requestBody = new
{
text = "Test",
from = new { textFrom = "+15551212" },
to = new object[] { new { sendTo = "+15551231" } }
};
var response = await extension.Sms().Post(requestBody); //Can't get past this point.
}