This ca be done by drawing a rectangle or polygon in the view, afterwards select it with the selection tool and then right click in the view and choose 'WKT from Geometry'. Copy the text.
Then you can go into the other scene and select from the context menu 'geometry from WKT'. Paste the text.
In the statistics tools you can choose the geometry as mask:
You can collocate both products but as suggested by SNAP you need to resample first. Due to the incomplete of the multi-resolution products. But actually it is not a big deal.
Resample both to the resolution of the S8 band, then create a subset which contains only the S8 band (you can do spatially tailor the product too), then do the collocation.
If you only save the last resulting product to disk it will not take much time.