test.cs

string xmlZugferd = xRechnung.CreateXML();
	
// load Xml file
string metaData = File.ReadAllText("metadata.xml");

TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings();

// create a new embedded file
var zugferdInvoice = new TXTextControl.EmbeddedFile(
   "ZUGFeRD-invoice.xml",
   Encoding.UTF8.GetBytes(xmlZugferd),
   metaData);

zugferdInvoice.Description = "ZUGFeRD-invoice";
zugferdInvoice.Relationship = "Alternative";
zugferdInvoice.MIMEType = "application/xml";
zugferdInvoice.LastModificationDate = DateTime.Now;

// set the embedded files
saveSettings.EmbeddedFiles = new TXTextControl.EmbeddedFile[] { zugferdInvoice };