test.cs

private void InsertFoldingMark(double top, int width, Color color, HeaderFooter headerFooter, TextControl textControl)
{
	textControl.PageUnit = MeasuringUnit.Twips;

	// create a new drawing control
	TXTextControl.Drawing.TXDrawingControl drawing =
		new TXTextControl.Drawing.TXDrawingControl(3000, 1000);

	// create a new line shape object
	TXTextControl.Drawing.Shape shape =
		new TXTextControl.Drawing.Shape(TXTextControl.Drawing.ShapeType.Line);

	// set the color and border width
	shape.ShapeOutline.Color = color;
	shape.ShapeOutline.Width = 20;
	shape.Angle = -45;
	shape.Movable = false;
	shape.Sizable = false;

	// add the shape to the drawing control
	drawing.Shapes.Add(
		shape,
		TXTextControl.Drawing.ShapeCollection.AddStyle.Fill);
	shape.Size = new Size(width, width);

	// create a new drawing frame object from the created drawing control
	TXTextControl.DataVisualization.DrawingFrame frame =
		new TXTextControl.DataVisualization.DrawingFrame(drawing);

	frame.Sizeable = false;
	frame.Moveable = false;

	var leftOffset = textControl.Sections[headerFooter.Section].Format.PageMargins.Left - MmToTwips(12);
	var topOffset = textControl.Sections[headerFooter.Section].Format.PageMargins.Top;

	// add the frame to the document
	headerFooter.Drawings.Add(
		frame, 
		new Point(-(int)leftOffset, 
		(int)(MmToTwips(top) - topOffset)), 
		FrameInsertionMode.BelowTheText | FrameInsertionMode.FixedOnPage);
}

public static double MmToTwips(double mm)
{
	const double mmPerInch = 25.4;
	const double twipsPerInch = 1440;

	double inches = mm / mmPerInch;
	double twips = inches * twipsPerInch;
	return twips;
}