We only provide Webhook from MODE cloud to call Smart Module end point. And so far, Webhook cannot add custom headers or change HTTP method other than POST.
I guess you are talking about the case where some web server as Smart Module already defined the API end point and it doesn't match the method, schema or header etc... right?
There are two approaches you can do.
1, Run your web service to receive Webhook, convert the schema, change the method and make another HTTP request. Actually, Initial State example takes this approach. This Node.js service only converts the request to another HTTP call.
2, Use scripting service like scriptr.io or AWS Lambda. You can read this doc