There should be a Siri icon on the left. But it just shows "Add to Siri" text and there is an empty space on the left. Tested in Xcode 10 beta 4 with iOS 12 beta 4 (both with device & simulator).
let addToSiriButton = INUIAddVoiceShortcutButton(style: .whiteOutline)
I just created the button like above and add it to view hierarchy. Am I missing something else?