There is a built in method called UploadValues that can send HTTP POST (or any kind of HTTP methods) AND handles the construction of request body (concatenating parameters with "&" and escaping characters by url encoding) in proper form data format:
using(WebClient client = new WebClient())
{
var reqparm = new System.Collections.Specialized.NameValueCollection();
reqparm.Add("param1", "<any> kinds & of = ? strings");
reqparm.Add("param2", "escaping is already handled");
byte[] responsebytes = client.UploadValues("http://localhost", "POST", reqparm);
string responsebody = Encoding.UTF8.GetString(responsebytes);
}