When using a Webhook in an Action Type, you must provide all configured webhook parameters opposite to when testing the respective webhook in data connections, where Optional parameters can be marked as “empty”.
Since the webhook is using in my case an outbound application with OAuth2 code grant, I cant use the webhook in functions. Apparently, there is the workaround of defining None parameters, however I think it should be possible in the action type directly, to assign Null / Undefined values.