How to find link label position

How could i find link label upper bound area and label lower bound area from label position on link for example

as shown in this diagram i just want to find the upper part of link label so that i can add this node before C


and using that i get the bottom part to so that i can able to add in bottom of C as shown in this figure

Once you have a reference to a specific label of a particular Link, you can call GraphObject.getDocumentPoint on it, passing go.Spot.Center as the argument. That will return its center point in document coordinates.