Modify request headers per request C# HttpClient PCL

Yes, you can create a new HttpRequestMessage, set all the properties you need to, and then pass it to SendAsync.

var request = new HttpRequestMessage() {
   RequestUri = new Uri("http://example.org"),
   Method = HttpMethod.Post,
   Content = new StringContent("Here is my content")
}
request.Headers.Accept.Add(...);  // Set whatever headers you need to

var response = await client.SendAsync(request);