Sending PDF base64 using POST web request - api

Question

I have a task which I need to upload a pdf converted to base64 string to Http post. I tried it using refit before but I failed to upload it. Now, I am trying a difference approach.
I used the web request POST method, First I converted the PDF to base64 but on the part that I call the stream.write method, I'm having an error since the file is already converted to base64 string and the stream.write paramater is byte array.
Also, how can I add these to the body using web request?
"mime": "application/pdf",
"data": "base64-data="
string pdfBase64;
const string pdfFileName = "file.pdf";
using (var pdfStream = await FileSystem.OpenAppPackageFileAsync(pdfFileName))
{
using (var pdfReader = new StreamReader(pdfStream))
{
var fileContents = await pdfReader.ReadToEndAsync();
pdfBase64 = Convert.ToBase64String(Encoding.UTF8.GetBytes(fileContents));
}
}
WebRequest request = WebRequest.Create("https://goodmorning-axa-dev.azure-api.net/upload");
request.Method = "POST";
request.ContentLength = pdfBase64.Length;
request.ContentType = "application/json";
request.Headers.Add("x-axa-api-key", apiKey);
Stream stream = request.GetRequestStream();
stream.Write(pdfBase64, 0, pdfBase64.Length);
stream.Close();
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
stream = response.GetResponseStream();
StreamReader sr = new StreamReader(stream);
Toast.MakeText(this, sr.ReadToEnd(), ToastLength.Short).Show();
sr.Close();
stream.Close();