Nevermind, I've found a solution by changing the parent name to the field group name.

 

				"operation": "insert",
				"name": "My Detail",
				"values": {
					"itemType": 2,
					"markerValue": "added-detail"
				},
				"parentName": "My Field Group",
				"propertyName": "items",
				"index": 1

Hello,

Could you please elaborate a bit on your business task as it's not quite clear what should be done?

Thank you in advance!

Best regards,

Anastasiia