Customer.cs

[HttpPost]
public string MergeDocument([FromBody] MergeDocument MergeDocument)
{
    // create some merge data rows
    List<Customer> lActualData = new List<Customer>() { 
        new Customer()
        {
            Company = "Text Control, LLC",
            Firstname = "Tim",
            Name = "Typer"
        },
        new Customer()
        {
            Company = "Text Control GmbH",
            Firstname = "Bernie",
            Name = "Bold"
        }
    };

    byte[] baResults;

    // create a new ServerTextControl instance to connect to MailMerge
    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();

        // load the template
        tx.Load(Convert.FromBase64String(MergeDocument.Document),
            TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        // connect MailMerge
        MailMerge mm = new MailMerge();
        mm.TextComponent = tx;

        // merge data into template
        mm.MergeObjects(lActualData);

        // save results
        tx.Save(out baResults, TXTextControl.BinaryStreamType.InternalUnicodeFormat);
    }

    return Convert.ToBase64String(baResults);
}