test.cs

// load a document
textControl1.Load("sample.html", TXTextControl.StreamType.HTMLFormat);

// create a MatchCollection
MatchCollection matchedFields = textControl1.Find(@"#(.*)#", RegexOptions.None);

// loop through all matches
foreach (Match match in matchedFields) {
  // select the text
  textControl1.Select(match.Index, match.Length);

  // create a new field based on the text
  TXTextControl.DocumentServer.Fields.MergeField mergeField =
    new TXTextControl.DocumentServer.Fields.MergeField() {
      Text = textControl1.Selection.Text,
      Name = textControl1.Selection.Text.Trim('#')
    };

  // enable highlighting and the second input position
  mergeField.ApplicationField.DoubledInputPosition = true;
  mergeField.ApplicationField.HighlightMode = TXTextControl.HighlightMode.Activated;

  // remove the text
  textControl1.Selection.Text = "";

  // insert the field at the current input position
  textControl1.ApplicationFields.Add(mergeField.ApplicationField);
}