POST a string to Web API controller in ASP.NET 4.5 and VS 2012 RC

I am new to WebAPI and trying to learn it. I have an WebAPI controller to which I am trying to POST a string using WebClient from my Unit Test.

I am posting a string to my WebAPI using following code below.

using (var client = new WebClient())
{
   client.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
   var result = client.UploadString(_webapiUrl, "POST", "hello");
}

Here is my controller.

[HttpPost]
public byte[] Post(string value)
{
   // Do something with value
}

I can hit a break point on my controller, but it doesn't seem to POST any string and I always get NULL value. What should I do to get the value ?

Thanks