i'm getting... {"errors":{"fulfillment":"can't be blank"}}
URL and POST data looks correct to me.
URL = https://shipping-tests.myshopify.com/admin/orders/176097097/fulfillments.json
post data is
{"fulfillment": {
"tracking_url": "http://www.google.com/search?q=1ZE7R7540391036916",
"tracking_number": "1ZE7R7540391036916",
"notify_customer": false,
"tracking_company": "ups"
}
}