test.cs

private static void ReplaceWithMergeField(ServerTextControl textControl, FieldInfo field)
{
	textControl.Select(field.StartIndex, field.EndIndex - field.StartIndex);
	var selectionText = textControl.Selection.Text;

	var mergeField = new MergeField
	{
		Name = field.FieldName,
		Text = selectionText,
		ApplicationField =
		{
			DoubledInputPosition = true,
			HighlightMode = HighlightMode.Activated
		}
	};

	textControl.Selection.Text = string.Empty;
	textControl.ApplicationFields.Add(mergeField.ApplicationField);
}