Many thanks Dave, that worked.!
FYI - this is how the whole ‘Event Properties’ code section looks like in the end:
{% assign firstFulfillmentOrder = getOrderDataForeachitem.fulfillmentOrders | first %}
{% assign firstFulfillment = firstFulfillmentOrder.fulfillments | first %}
{% assign firstTrackingInfo = firstFulfillment.trackingInfo | first %}
{
"deliveryStatus": "deliveryFailed",
"customerEmail": "{{getOrderDataForeachitem.email}}",
"orderNumber": "{{getOrderDataForeachitem.name}}",
"trackingNumber": "{{ firstTrackingInfo.number }}",
"trackingUrl": "{{ firstTrackingInfo.url}}"
}