I eventually got the action payload definition to actually update with my newly added parameter.

The new parameter was "query". I added the query (copied and pasted encoded from the list) and then, foolishly expected it to just work. But it doesn't.

I also tried "filter". I've tried adding it as reference to the payload map, I've tried....EVERYTHING else but the list doesn't filter. I thought I'd cracked it when I realised that I'd added it to the Target Payload Mapping as "type": "EVENT_PAYLOAD_BINDING" but I changed it to JSON literal and it still doesn't filter.

Is there a special trick to this?

I get my list as specified here:

But it's the entire list. How do I get it to see the filter?