A Geoprocessing Service is an ideal candidate for a webhook handler since ArcGIS server implements the necessary web server endpoint and the scripting can easily access ArcGIS data and functions.
However right now a GP service requires structured form values for each parameter in the POST body and won't pass thru the JSON body from the current webhook. I've tried a few options and the webhook fires the GP Service without issue ie:
https://myserver.com/arcgis/rest/S123Handlers/GPServer/MyWebHookScript/execute
but the JSON data is never passed thru.
A simple option 'POST as form values' and either:
1. POST the entire json to a single named parameter (eg 'eventData')
2. POST each selected event data to it's own named parameter: SurveyInfo, UserInfo, etc..
If this was an option, then the GP script can easily be created to match the correct name(s) with string/text type and then parsed in the script.