test.cs

[HttpGet]
public IActionResult MergeDocument()
{

    byte[] bDocument;

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

        tx.Create();

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

        // create the mail merge engine
        using (TXTextControl.DocumentServer.MailMerge mm = new TXTextControl.DocumentServer.MailMerge())
        {
            // connect to ServerTextControl instance
            mm.TextComponent = tx;

            // merge data
            var jsonData = System.IO.File.ReadAllText("App_Data/data.json");
            mm.MergeJsonData(jsonData);
        }

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

    return Ok(bDocument);
}