How to query by array of objects in Contentful

I have an content type entry in Contentful that has fields like this:

"fields": {
                "title": "How It Works",
                "slug": "how-it-works",
                "countries": [
                    {
                        "sys": {
                            "type": "Link",
                            "linkType": "Entry",
                            "id": "3S5dbLRGjS2k8QSWqsKK86"
                        }
                    },
                    {
                        "sys": {
                            "type": "Link",
                            "linkType": "Entry",
                            "id": "wHfipcJS6WUSaKae0uOw8"
                        }
                    }
                ],
                "content": [
                    {
                        "sys": {
                            "type": "Link",
                            "linkType": "Entry",
                            "id": "72R0oUMi3uUGMEa80kkSSA"
                        }
                    }
                ]
            }

I'd like to run a query that would only return entries if they contain a particular country.

I played around with this query:

https://cdn.contentful.com/spaces/aoeuaoeuao/entries?content_type=contentPage&fields.countries=3S5dbLRGjS2k8QSWqsKK86

However get this error:

The equals operator cannot be used on fields.countries.en-AU because it has type Object.

I'm playing around with postman, but will be using the .NET API.

Is it possible to search for entities, and filter on arrays that contain Objects?

Still learning the API, so I'm guessing it should be pretty straight forward.

Update:

I looked at the request the Contentful Web CMS makes, as this functionality is possible there. They use query params like this:

filters.0.key=fields.countries.sys.id&filters.0.val=3S5dbLRGjS2k8QSWqsKK86

However, this did not work in the delivery API, and might only be an internal query format.

Solution

Figured this out. I used the following URL:

https://cdn.contentful.com/spaces/aoeuaoeua/entries?content_type=contentPage&fields.countries.sys.id=wHfipcJS6WUSaKae0uOw8

Note the query parameter fields.countries.sys.id