Hey, @AdvantageDev 
Thanks for the question!
I want to make sure we’re on the same page. You have a Private App set up along with your custom contact property? And you’re navigating in-app to Private Apps> Your App. Webhooks tab?
I created a custom contact property called Help AD `help_ad` and set up a webhook for it in-app
Can you create a simple test property similar to what I did and find it in the Webhook settings for Private Apps?
Thank you! — Jaycee