Ok, I was able to get a Photon posting to a dynamic webhook URL (this particular example is for posting to a Slack channel):
Code:
Spark.publish("log_info", "{\"my-log\": \"Yo!\", \"my-url\":\"a/b/c\"}", 60);
Webhook definition:
{
"eventName": "log_",
"url": "https://hooks.slack.com/services/{{my-url}}",
"requestType": "POST",
"json": {
"username": "{{SPARK_CORE_ID}}",
"text": "{{my-log}}"
}
}
Hope that works for you, too!