400 Bad Request when posting to Rest API over https - rest

Question

I'm trying to post a file in a json object to an external rest api over https. I have confirmed the json object is formatted correctly, do I have to do anything special to post to a rest api over https? I'm using the answer found over here as a guide: How to post JSON to the server?
private static void PostDatatoFTP(string FileName,
string fileString, string centerCode, string fileType) {
try {
byte[] plainTextBytes = Encoding.ASCII.GetBytes(fileString);
string base64File = Convert.ToBase64String(plainTextBytes);
FileInfo fileInfo = new FileInfo {
FileData = base64File,
FileName = FileName,
FileType = fileType,
FileVersion = _fileVersion
};
FileInfo[] transmitFileInfo = new FileInfo[1];
transmitFileInfo[0] = fileInfo;
Json jsonObject = new Json {
RequestType = _RequestType,
APIVersion = _apiVersion,
SubmissionId = Guid.NewGuid().ToString(),
UserId = _ftpUsername,
Password = _ftpPassword,
Vendor = _vendor,
CenterCode = centerCode,
FileInfo = transmitFileInfo
};
var json = JsonConvert.SerializeObject(jsonObject);
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(_uploadPath);
request.Method = "POST";
request.ContentType = "application/json";
using (var streamWriter = new StreamWriter(request.GetRequestStream())) {
streamWriter.Write(json);
}
var httpResponse = (HttpWebResponse)request.GetResponse();
using (var streamReader = new StreamReader(httpResponse.GetResponseStream() ?? throw new InvalidOperationException())) {
var result = streamReader.ReadToEnd();
Console.WriteLine(result);
}
}
catch (WebException e) {
Console.WriteLine(e.Message);
String status = ((HttpWebResponse)e.Response).StatusDescription;
Console.WriteLine(status);
}
catch (Exception ex) {
Console.WriteLine(ex.Message);
}
}