test.cs

[HttpPost]
public IActionResult Merge([FromBody] Content results)
{
    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();
        tx.Load(Convert.FromBase64String(results.Data), TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        using (MailMerge mm = new MailMerge())
        {
            mm.TextComponent = tx;

            mm.FieldMerged += Mm_FieldMerged;

            // load json data
            string json = System.IO.File.ReadAllText("App_Data/data.json");
            // merge data
            mm.MergeJsonData(json, true);
        }

        tx.Save(out byte[] data, TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        return Ok(Convert.ToBase64String(data));
    } 
}

private void Mm_FieldMerged(object sender, MailMerge.FieldMergedEventArgs e)
{
    e.MergedField = e.MergedField;
}