The fulfillment data sent to your webhook URL is like below:

{
	"responseId": "4a68fc8r...",
	"queryResult": {
		"queryText": "fullfilment",
		"parameters": [],
		"allRequiredParamsPresent": true,
		"fulfillmentText": "Example",
		"fulfillmentMessages": [{
			"text": {
				"text": ["Response"]
			}
		}],
		"outputContexts": [{
			"name": "projects/rpaix...",
			"parameters": {
				"no-input": 0,
				"no-match": 0
			}
		}],
		"intent": {
			"name": "projects/rpaix...",
			"displayName": "Fullfilment"
		},
		"intentDetectionConfidence": 1,
		"languageCode": "en"
	},
	"originalDetectIntentRequest": {
		"payload": {
			"AiX": {
				"conversation_id": "3002",
				"user_id": "3777"
			}
		}
	},
	"session": "projects/example/agent/sessions/3777-3002"
}

The payload and session fields contain the AiX user ID and conversation ID.