I have attached a sample project showing how to use Wisej.Web.Widget directly. As I mentioned in the linked post, there is no need for an extension unless you want to turn a widget into a full fledged Wisej control. In any case, the first step is always to use it directly and get familiar with the widget.
This is the vis.js in Wisej designer. The sample project is attached. Look at the InitScript content, I simply copied and pasted their samples and changed container = document.getElementById(…) to container = this.container.