I am using an iPhone 5S and I just updated to iOS 10. I enabled the Siri support towards WhatsApp in Settings > Siri > App Support.
However, when I use Siri and ask, for example "Send a message to XXX using WhatsApp saying Hello", Siri tells me "Sorry, you'll need to continue in the app".
It seems like the integration has not been enabled yet. Does someone know how to solve this issue?