Code

using Shape = MindFusion.Diagramming.Wpf.Shape;
...
ElementTemplate[] textArea = new ElementTemplate[]
{
	new LineTemplate(-50, 100, 150, 100),
	new LineTemplate(150, 100, 150, 150),
	new LineTemplate(150, 150, -50, 150),
	new LineTemplate(-50, 150, -50, 100)
};

Shape textBelow = new Shape(
	Shapes.Rectangle.Outline, // copy the Rectangle outline definition
	null, // no decorations
	textArea,
	FillRule.Nonzero,
	"TextBelow");

ShapeNode node = new ShapeNode(diagram);
node.Shape = textBelow;
node.Text = "put it above or below the node";
diagram.Nodes.Add(node);