// If you're going to be uploading a file, you'll need to first read it in as a byte array
FileStream fs = new FileStream("c:\\test.jpg", FileMode.Open, FileAccess.Read);
byte[] data = new byte[fs.Length];
fs.Read(data, 0, data.Length);
fs.Close();

// Generate post objects
Dictionary<string, object> postParameters = new Dictionary<string, object>();

// Adding a file as the post parameter Be sure to add the content-type as well
postParameters.Add("file", new FileParameter(data, "test.jpg", "image/jpeg"));

// Adding a normal name/value post parameter
postParameters.Add("name", "FileName");

// Create request and receive response
string postURL = "http:https://linktotheplaceyouwanttopost.com/link.html";

// Create an HttpWebResponse object and use either the MultipartFormDataPost WITH Basic HttpAuth params or without
HttpWebResponse webResponse = FormUpload.MultipartFormDataPost(postURL, "UserName", "Password", postParameters);

// Process response
StreamReader responseReader = new StreamReader(webResponse.GetResponseStream());
string fullResponse = responseReader.ReadToEnd();
webResponse.Close();
// Do something with fullResponse. This may contain xml, json, or whatever the API you're posting to is using.

This class is based heavily on a blog post by Brian Grinstead which you may find here:
[http:https://www.briangrinstead.com/blog/multipart-form-post-in-c][1]

I've taken his example and blown it out a bit to add some functionality such as basic http auth functions.