How to send XML content with HttpClient.PostAsync?

I am trying to fulfill this rest api:

public async Task<bool> AddTimetracking(Issue issue, int spentTime)
{
    // POST /rest/issue/{issue}/timetracking/workitem
    var workItem = new WorkItem(spentTime, DateTime.Now);
    var httpContent = new StringContent(workItem.XDocument.ToString());
    var requestUri = string.Format("{0}{1}issue/{2}/timetracking/workitem", url, YoutrackRestUrl, issue.Id);
    var respone = await httpClient.PostAsync(requestUri, httpContent);
    if (!respone.IsSuccessStatusCode)
    {
        throw new InvalidUriException(string.Format("Invalid uri: {0}", requestUri));
    }

    return respone.IsSuccessStatusCode;
}

workItem.XDocument contains the following elements:

<workItem>
  <date>1408566000</date>
  <duration>40</duration>
  <desciption>test</desciption>
</workItem>

I am getting an error from the API: Unsupported Media Type

I really have no idea how to resolve this, help is greatly appreciated. How do I marshall an XML file via a HTTP POST URI, using HttpClient?

You might want to set the mediaType in StringContent like below:

var httpContent = new StringContent(workItem.XDocument.ToString(), Encoding.UTF8, "text/xml");

OR

var httpContent = new StringContent(workItem.XDocument.ToString(), Encoding.UTF8, "application/xml");

You could use

var respone = await httpClient.PostAsXmlAsync(requestUri, workItem);

https://msdn.microsoft.com/en-us/library/system.net.http.httpclientextensions_methods