How can I achieve Table Grouping with Custom API Data Source?

I managed to enable the grouping functionality through grouping=true option (see screenshot)

options: {
   grouping: true
}

But upon sending a request against /select, it sends a JSON structure that doesn't seem to help.

{
"type": "select",
"query": {
"aggs": {
"by": {
"rows": [
    {
       "uniqueName": "My Field.groups"
    }
]
},
"values": [
{
"func": "sum",
"field": {
"uniqueName": "another_field"
}
}
]
}
},
"page": 0,
}

How can this be achieved with Custom API Data Source?

Thanks