Code

class MyLink : DiagramLink
{
	public MyLink(Diagram parent) : base(parent)
	{
		MyProperty = "go there";
	}

	public override void Draw(
		System.Windows.Media.DrawingContext graphics,
		MindFusion.Diagramming.Wpf.RenderOptions options)
	{
		base.Draw(graphics, options);
		var width = Parent.MeasureString(MyProperty, this, 1000).Width + 10;
		var bounds = new Rect(
			EndPoint, new Size(width, 100));
		if (EndPoint.X > StartPoint.X)
			bounds.Offset(-bounds.Width, 0);
		bounds.Offset(-Bounds.X, -Bounds.Y);
		Parent.DrawStyledText(graphics, MyProperty, this, bounds);
	}

	protected override void SaveToXml(XmlElement xmlElement, XmlPersistContext context)
	{
		base.SaveToXml(xmlElement, context);
		context.WriteString(MyProperty, "MyProperty", xmlElement);
	}

	protected override void LoadFromXml(XmlElement xmlElement, XmlPersistContext context)
	{
		base.LoadFromXml(xmlElement, context);
		MyProperty = context.ReadString("MyProperty", xmlElement);
	}

	public string MyProperty { get; set; }
}

public partial class MainWindow : Window
{
	public MainWindow()
	{
		InitializeComponent();

		Diagram.RegisterItemClass(typeof(MyLink), "my:MyLink", 100);

		diagram.Behavior = Behavior.Custom;
		diagram.CustomLinkType = typeof(MyLink);

		diagram.AllowUnanchoredLinks = false;
		diagram.SnapToAnchor = SnapToAnchor.OnCreateOrModify;
		.....