Cool that you are trying to use the widget-base in ES6 to build a widget. In order to get the value of this attribute, you will need to first get the context object.
I have an example here:
A more sophisticated example is the Leaflet Maps widget, you can find examples here: https://github.com/mendix/Leaflet/blob/master/src/Leaflet/widget/Leaflet.js
mxobject.get(key) is correct. In order to get a reference to an mxobject you need either to do an mx.data.get first or get the object from the context i.e. this._contextObj
I always find it usefull to take a look at how other widget work when I am struggling with some part of the mendix client API. See for example - https://github.com/mendix/BooleanSlider/blob/master/src/BooleanSlider/widget/BooleanSlider.js