test.cs

[HttpPost]
[Route("SaveDocument")]
public bool SaveDocument([FromBody] DocumentData documentData)
{
  using (var tx = new ServerTextControl())
  {
    tx.Create();
    tx.Load(Convert.FromBase64String(documentData.Document), BinaryStreamType.InternalUnicodeFormat);

    foreach (FormField formField in tx.FormFields)
    {
      // access the form field data
    }
  }

  return true;
}