C# Sending SMS thows System.UriFormatException: 'Invalid URI: The format of the URI could not be determined.'

Trying to send a simple SMS with this code it throws

public void SendText()

{

//This throws System.UriFormatException: 'Invalid URI: The format of the URI could not //be determined.' at this point

var sdk = new SDK(appkey, appSecret, "SDK.Server.Sandbox", " MyApp", "1.0.0");

var jsonString = "{ "text": "TryThis RC!", "from": { "phoneNumber": "+15558881212" }, "to": [{ "phoneNumber": "+18008675309" }] }";

var request = new Request("/restapi/v1.0/account/~/extension/~/sms", jsonString);

var response = sdk.Platform.Post(request);

}

What am I doing wrong?