Lambda event filter for Json array

In EventBridge, your custom payload is always included in the "detail" field, so your actual event will look like:

{
  "detail-type": "MyCustomEvent",
  "source": "aws.partner/salesforce.com/xxxxxxxxxxxx/xxxxxxxxxxx",
  "account": "12345678921",
  "time": "2022-04-06T20:24:01Z",
  "resources": [],
  "detail": {
    "MyKey": "Value",
    "Number": 12,
    "References": [{
      "Key1": 14
    }, {
      "Key2": 14
    }]
  }
}

Your rule should look like this:

{
  "detail": {
    "References": {
      "Key2": [14]
    }
  }
}