If you have already set the "grid" keyword in your request, you can add the "area" : "coordinates" keyword. You can set pre-defined area regions e.g. 'europe' or set the area using coordinates North/West/South/East.
See the MARS Legacy Post-processing keywords for more information.
Some examples:
"area": "europe",
#area: N/W/S/E #europe "area": "75/-20/10/60", #africa "area": "40/-20/-40/60",