backend.cs

[ApiController]
[Route("[controller]")]
public class TextControlController : ControllerBase
{
    [HttpPost]
    [Route("MergeDocument")]
    public MergedDocument MergeDocument([FromBody] MergedDocument Document)
    {
        byte[] data;

        // create dummy merge data 
        Customer customer = new Customer() { Firstname = "Klaus", Name = "Klaasen" };

        // new ServerTextControl instance
        using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
        {
            tx.Create();

            // load the document from the payload
            tx.Load(Convert.FromBase64String(Document.Document), 
                TXTextControl.BinaryStreamType.InternalUnicodeFormat);

            TXTextControl.DocumentServer.MailMerge mm = 
                new TXTextControl.DocumentServer.MailMerge();
            mm.TextComponent = tx;

            mm.MergeObject(customer); // merge the data into template

            // save document into byte array
            tx.Save(out data, TXTextControl.BinaryStreamType.InternalUnicodeFormat);

            // return the document to the client
            return new MergedDocument() { Document = Convert.ToBase64String(data) };
        }
    }
}