var contentFormat = "xml";
var packetsToMarkAsCompleted = "<messages>"+
"<message><id>asdasdasd1</id><receiptHandle>abvabc1</receiptHandle>"+
"<message><id>asdasdasd2</id><receiptHandle>abvabc2</receiptHandle>"+
"<message><id>asdasdasd3</id><receiptHandle>abvabc3</receiptHandle>"+
var client = new RestClient("https://api.portal.dynamicsmobile.com/mobile/erp/pull/"+appArea+"/done?format="+contentFormat);
var request = new RestRequest(Method.POST);
request.AddHeader("x-api-key",apiKey);
request.AddHeader("Content-Type", "text/plain");
request.AddParameter("application/"+contentFormat, packetsToMarkAsCompleted, ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);