If I want to open a Web App (built with WebAppBuilder) using a "launch in context" call from another app, then it seems I can use a given set of URL Parameters to achieve this: Use URL parameters—Portal for ArcGIS | ArcGIS Enterprise .
However, if I want to parameterise the layers to make visible, the above URL doesn't identify a means by which I can achieve this. (This is unlike parameterised calls to the Portal's Web Map Viewer application, which does include layer visibility among its parameters: URL parameters to modify maps—Portal for ArcGIS | ArcGIS Enterprise).
Given that I want to use a Web App built with WebAppBuilder, any ideas as to how, if I were to include a layer list in my url querystring, I could interpret this and only show the requested layers? Given that the normal means of extension is by means of a Widget, I'm imagining that I might be able to use a Widget that is configured to open at startup? I'm not sure how I would then access the querystring, however? Or someone might have a better approach to the problem?