How can I set my button like this?

What did you try? (Leave out irrelevant details in your code.)

$(“Button”,{ click: partContextMenu, alignment: go.Spot.TopRight },
$(go.Shape, “BpmnEventConditional”, { width: 6, height: 6 })

That’s good. But what does partContextMenu do? It should do something like:

   click: function(e, obj) {
     var cmtool = e.diagram.toolManager.contextMenuTool;
     e.diagram.currentTool = cmtool;  // start running the tool,
     cmtool.isActive = true;  // make it active, then show the menu
     cmtool.showContextMenu(obj.part.contextMenu, obj.part);

AWESOME!! Thank You! walter. ^_^