test.cs

[HttpGet]
public IActionResult AddWatermark()
{

    byte[] bDocument;

    // create a ServerTextControl
    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) { 

        tx.Create();
        tx.PageUnit = TXTextControl.MeasuringUnit.Twips;

         // load the template
        tx.Load("App_Data/nda.tx", TXTextControl.StreamType.InternalUnicodeFormat);

        AddWatermark(tx, "App_Data/draft.svg");

        // save in the internal format
        tx.Save(out bDocument, TXTextControl.BinaryStreamType.InternalUnicodeFormat);
    }

    return Ok(bDocument);
}