test.cs

public IActionResult Index()
{
    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();

        // adding static text
        TXTextControl.Selection sel = new TXTextControl.Selection();
        sel.Text = "Welcome to Text Control\r\n";
        sel.Bold = true;

        tx.Selection = sel;

        // adding merge fields
        TXTextControl.DocumentServer.Fields.MergeField mergeField =
          new TXTextControl.DocumentServer.Fields.MergeField()
          {
              Text = "{{company}}",
              Name = "company",
              TextBefore = "Company name: "
          };

        tx.ApplicationFields.Add(mergeField.ApplicationField);

        // merge fields with MailMerge engine
        using (TXTextControl.DocumentServer.MailMerge mailMerge =
          new TXTextControl.DocumentServer.MailMerge())
        {
            mailMerge.TextComponent = tx;
            mailMerge.MergeJsonData("[{\"company\": \"Text Control, LLC\" }]");
        }

        byte[] baPdf;
        tx.Save(out baPdf, TXTextControl.BinaryStreamType.AdobePDF);

        ViewBag.Document = baPdf;
    }

    return View();
}