You can add a parameter GEOMETRY as in this example:
https://services.sentinel-hub.com/ogc/wms/<INSTANCE_ID>?SERVICE=WMS&REQUEST=GetMap&VERSION=1.3.0&LAYERS=TRUE_COLOR&MAXCC=20&WIDTH=640&HEIGHT=640&CRS=EPSG:4326 &BBOX=-7.885,38.540,-7.870,38.560& GEOMETRY=POLYGON(( -7.877244 38.546511, -7.876377 38.547818, -7.871950 38.546125, -7.872611 38.545023, -7.871241 38.544475, -7.869831 38.544560, -7.866011 38.550445, -7.872323 38.552895, -7.874112 38.551451, -7.877110 38.552537, -7.878791 38.552976, -7.879413 38.553099, -7.880600 38.553320, -7.881314 38.553126, -7.882678 38.552762, -7.883951 38.552667, -7.885064 38.552160, -7.885370 38.549346, -7.877244 38.546511))
It is recommended to simplify geometry before passing it as a parameter to avoid exceeding maximum number of characters in URL.
If one wants to have a transparent background outside of the clipped geometry, she can use "FORMAT=image/png&TRANSPARENT=1" parameters in the call. Do note however that png images are larger in size and will therefore take longer to load.