Code

private void diagram_LinkCreated(object sender, LinkEventArgs e)
{
	DiagramLink link = e.Link;
	if (link.Origin == link.Destination)
	{
		link.TextColor = Color.Transparent;
		link.CustomDraw = CustomDraw.Additional;
		link.Text = "test test";
	}
}

private void diagram_DrawLink(object sender, DrawLinkEventArgs e)
{
	if (e.Link.Origin != e.Link.Destination)
		return;

	int selfLoopCounter = -1;
	foreach (DiagramLink link in e.Link.Origin.OutgoingLinks)
	{
		if (link.Origin == link.Destination)
			selfLoopCounter++;
		if (link == e.Link)
			break;
	}
	PointF p = e.Link.ControlPoints[1];
	p.Y -= selfLoopCounter * 3;
	e.Graphics.DrawString(e.Link.Text, e.Link.Font, Brushes.Black, p);
}