Hi.
You can't create a popup on the node or near it, if you don't know where to put it or where the node is located.
I guess you already have a node somewhere and so you want to show popup after chart is loaded.
First, you need to use chart.getNode(yourNodeID)
API method:
https://zoomcharts.com/developers/en/net-chart/api-reference/api.html#getNode
It will return the node object and there you will find coordinates of the node.
Then you can create your popup and position in using those coordinates.
If you have dynamic layout and indeed you want to do this after chart is loaded, then I suggest you to use setTimeout() as initially nodes will expand from the center.
Or you could use onChartUpdate event to reposition popup when the node is moving.