Accessing Web Messaging participant attributes in Architect flow

Architect & Flows
1

Could someone explain how to pull participant attributes set by Web Messaging into an Architect Inbound Message flow? I keep hitting a 400 Bad Request when mapping the data action. My current JSON payload looks like this:

{
 "type": "setParticipantAttributes",
 "attributes": {
 "ticketId": "{{webChat.tid}}"
 }
}

The flow runs on Genesys v2.0 but the attributes never populate. I need the exact data action syntax or REST endpoint to resolve this. I am on macOS 14 with Node 20. 1. Show the correct mapping. 2. Explain the endpoint.