Create a new webhook subscription

curl --request POST \
  --url https://api.shipbob.com/1.0/webhook \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "subscription_url": "https://mywebsite.com/shipbob/handler",
  "topic": "order_shipped"
}'

{
  "created_at": "2019-08-24T14:15:22Z",
  "id": 12345,
  "subscription_url": "http://example.com",
  "topic": "order_shipped"
}