Webhook .NET C# - Send Test Notification Bad Request

My goal is to leverage a webhook as middleware to send data from the Shopify Client to my external API application. I tried using the proxy app approach but it seems you can only bring data to Shopify’s client and not bring data from Shopify to an external backend.

Sending a test notification in the admin settings results in a Bad Request, although I’m using the ngrok url and subpath that would successfully hit my method using an ajax call. What is required to make this work? I’ve looked at documentation and would greatly appreciate help