One of my customers asks if it's possible to order a product directly from Siri like "order <amount> <brand> <product>". Brand is not important here since there will be a single brand but i think it's required to indicate the app. In speech framework documentation, i wasn't able to find a exactly fitting usage. I thought about some workarounds but we all know how Apple doesn't like workarounds and rejects the apps. So, is there a proper method to do this which is approved by Apple?
The proper method to use Siri for ordering a product
See my reply in the SiriKit thread, which is about how to integrate your app with Siri and fill out a feature enhancement:
https://forums.developer.apple.com/thread/75395
Since SiriKit currently does not support your needs, you should use the Speech Recognition APIs directly in your app by having the user tap on a button to trigger your speech recognition code. The iOS Speech Recogniton APIs transcribe the audio into text and you can then use natural language processing (NLP) to map that into your app's e-commerce APIs.
BTW even if SiriKit supported an e-commerce domain, you would still need to use NLP to figure out what the speaker ordered. NLP in iOS has been available since iOS 5 but check out the video and code examples from the WWDC 2017 session "Natural Language Processing and your Apps" https://developer.apple.com/videos/play/wwdc2017/208/
So even if you can't get the built-in benefits of the conversational UI and Siri trigger support, you can still provide a voice interface to your app's e-commerce API