You can implement that using a LinkLabel like this:

var endLabel = new LinkLabel
{
    Text = "this is the end",
    RelativeTo = RelativeToLink.ControlPoint,
    Index = -1,
    AutoArrange = true
};
link.AddLabel(endLabel); 

Negative Index value signifies a control point relative to the last one, where -1 is the last point. Set AutoArrange to automatically adjust the label so that it does not overlap nodes.

Alternatively, with AutoArrange disabled you could set label's position manually via the HorizontalOffset and VerticalOffset properties if you need it placed at a specific position. Offsets are relative to the point specified via Index and RelativeTo, e.g. the link's end point in this case.

I hope that helps,
Stoyan