To enroll, go to Software Update and tap the prompt at the bottom.
Device region needs to be set to US, language to US English, Siri also needs to be set to US. I was able to join the wait list from Canada after changing all those settings.
F that, they indiscriminately collected data from European websites, but still somehow won’t release the feature here. This is absolutely a dick move. I don’t understand how they think this is going to play out, but I can’t imagine it being good for anyone (mostly them).