SubsetOp with .shp

[marpet]

You can try the following approach for only a few shapefiles:

Subset several images on a same area

OK, now I got it. If you have imported the shapefile in the GUI. You can open a image view, display the shapefile and select the geometry. Then do a right-click on the image view and select from the context menu WKT from Geometry

or use an online conversion tool like: SHP to WKT Converter Online - MyGeodata Cloud

For snappy there is no good API yet. It is better to convert it upfront.