Delivery API - More from this Category Recommendations - Payload format

[ALangridge]

Hey all, getting stuck into the Target Delivery API and have found success in getting an API call working to retrieve basic results ('Most viewed', 'most purchased'). 

I'm aiming to take a step forward on this and start utilising category based popularity recommendations. I've setup an activity using the appropriate criteria and am able to see results by categories when I download the results CSV. 

https://experienceleague.adobe.com/en/docs/target/using/recommendations/criteria/base-the-recommenda... 

While reviewing this documentation, I can see that the entity.categoryId value must be passed as a parameter to the display mbox. 

I'm running into a bit of difficulty figuring out where this needs to be provided in the API call payload however, I'm referring to https://developer.adobe.com/target/implement/delivery-api/#operation/execute and its a touch sparse in it's detail. 

Is anyone able to provide some clarification on where this value should be provided? My assumption is that it would be via the parameters object within the mboxes array, but this does not seem to be getting the results I want.

{
    "environmentId": 1234,
    "context": {
        "channel": "web"
    },
    "property": {
        "token": '1234'
    },
    "execute": {
        "mboxes": [
            {
                "name": "moreFromCategory",
                "index": 0,
                "parameters": {
                    "entity.categoryId": "Category A"
                }
            }
        ]
    }
}