From  https://github.com/kiegroup/kogito-examples/tree/main/serverless-workflow-examples/serverless-workflow-expression-quarkus 

{
    "$schema": "http://json-schema.org/draft-04/schema#",
    "title": "Expression",
    "description": "Schema for expression test",
    "type": "object",
    "properties": {
        "numbers": {
            "description": "The array of numbers to be operated with",
            "type": "array",
            "items" : {
                "type" : "object",
                "properties" :

{                    "x" :

,
                   "y" : {"type":"number"}
                }
             }
        }
    },
    "required": ["numbers"]
}

This is valid, but for some reason is not rendering on dev ui