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.