For the first week I used my Apple Watch, it drove me nuts that I still had to tap the screen to confirm sending text messages I'd dictated via "Hey Siri". Then I decided to try dictating the button press. When presented with the confirmation button before sending a text message, saying "Send", which is most intuitive, doesn't work. However, saying "Hey Siri, Send" does. In fact, for any confirmation button that shows up while dictating into the watch, all you have to do do is say "Hey Siri" and then the name of the button.
"Hey Siri, Tell Daisy I'm in jail. Bring bail money."
"Hey Siri, Send."
It is strange that you have to preface every button press with "Hey Siri" and this behavior is different from the iPhone, which asks you to confirm and you just say "yes" or "confirm". The iPhone method is better. However, if you want to send a text message from your Apple Watch hands free, get ready to say "Hey Siri" a lot.
Turns out, the iDownload blog figured this out before I did and even made a clever video.