Is there any way to create custom trigger or maybe existing trigger to subscribe/unsubscribe and pass some config to external application - e.g. api_key for specific client.
I know we can create recipe with outbound filed but this does not looks like proper way from security perspective
If you want your app to access a client’s API key, I’d recommend implementing OAuth. It will allow your server to independently retrieve a user’s API key (instead of asking them to add it as an input field).
You can implement an Authorization URL and ask for the API key on that page. This request would go directly to your server instead of via the apps framework servers.