The topic where you posted is about GoDiagram, but I assume you are asking about GoJS, so I have moved it to be a separate topic.
On the arrowhead Shape, set segmentOffset: new go.Point(10, 0) after setting toArrow. You may need to use a different X value for the offset to get the visual effect that you want – I don’t know how your arrowhead and port are defined, so I am just guessing about the distance.